JP2002190300A - Lithium secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary cell which provides no big change in a cell characteristics such as capacity drop while exhibits excellent safety at over-charging. SOLUTION: This cell employs a spinel-structure lithium manganese compound oxide whose basic composition is LiMn2O4 as a positive electrode active material. The positive electrode active material contains a first positive electrode active material represented by a composition formula LiMxMn2-xO4 (M is at least one kind selected from among Li, Mg, Al, Ni, and Cu; 0<=x<=0.1), and a second positive electrode active material represented by a composition formula LiMyMn2-yO4 (x<y<=0.3).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery utilizing the occlusion / desorption of lithium, and more particularly to a lithium secondary battery excellent in safety.

[0002]

2. Description of the Related Art With the miniaturization of personal computers, video cameras, mobile phones, and the like, in the fields of information-related equipment and communication equipment, lithium secondary batteries are used as power sources for these equipments because of their high energy density. Has been put to practical use and has spread widely. On the other hand, in the field of automobiles, the development of electric vehicles is urgent due to environmental problems and resource problems, and lithium secondary batteries are being studied as power sources for electric vehicles.

At present, as a positive electrode active material of a lithium secondary battery, LiC is required to constitute a 4V-class secondary battery.
Lithium transition metal composite oxides such as oO ₂ , LiNiO ₂ and LiMn ₂ O ₄ are preferably used.
O _2, in addition to synthesis easy and handling is relatively easy, since it is excellent in charge-discharge cycle characteristics, a secondary battery using LiCoO ₂ as the positive electrode active material has become mainstream.

[0004] However, cobalt is a scarce resource, and a secondary battery using LiCoO ₂ as a positive electrode active material is difficult to cope with future mass production and enlargement in view of the power source for electric vehicles, and is expensive. Must also be very expensive. Therefore, a secondary battery using LiMn ₂ O ₄ having a spinel structure, which is abundant as a resource and contains inexpensive manganese as a constituent element, instead of cobalt as a positive electrode active material is expected.

In general, a lithium secondary battery uses a non-aqueous electrolyte using an organic solvent as an electrolyte serving as a constituent element. Therefore, sufficient consideration must be given to its safety. In particular,
When falling into an overcharged state, a phenomenon such as an increase in internal pressure due to decomposition of the electrolytic solution due to the overcharge reaction is caused.
A lithium secondary battery having high safety against overcharging has been desired since it leads to a phenomenon such as a temperature rise due to a chain of exothermic reactions.

As a technique for improving the safety of a lithium secondary battery, installing a safety valve in a battery case is a well-known technique. This safety valve releases internal gas when the battery internal pressure reaches a predetermined pressure, and exhibits a certain effect for improving safety. However, depending on overcharge conditions, such as a large overcharge current, the temperature of the battery continues to rise even after the safety valve is opened, and there is a possibility that the temperature may further fall due to a rapid chain reaction.

[0007] It is known that in the initial stage of the overcharged state, the electrolyte is decomposed on the surface of the positive electrode active material to generate gas. Therefore, as described in JP-A-4-345770, Li ₂ CO ₃ or the like is added into the positive electrode,
A technology for generating highly safe CO ₂ gas, promoting an increase in internal pressure at the early stage of overcharging to open a safety valve, and then trying to calm down the overcharging reaction has been studied.

However, since Li ₂ CO _{3 and the} like do not contribute to the charge / discharge reaction, the addition of such a substance into the positive electrode lowers the capacity of the battery, resulting in a high energy density. Therefore, the advantage of the lithium secondary battery is lost. Also, Li ₂ CO ₃
Although decomposition occurs at a battery voltage of about 4.8 V, an overcharge reaction may occur before decomposition depending on the type of the positive electrode active material, overcharge conditions, and the like, and this still leaves a problem.

[0009]

SUMMARY OF THE INVENTION The present inventor has studied the overcharge reaction of a lithium secondary battery using LiMn ₂ O ₄ having a spinel structure as a positive electrode active material, in particular, the overcharge reaction of a positive electrode active material. After a number of experiments, certain findings were obtained. Even when the spinel structure lithium manganese composite oxide has the same basic composition, a slight change in the composition causes a difference in the ease of desorption of lithium, and the timing at which decomposition of the electrolytic solution occurs in an overcharged state is different It will be. If this phenomenon is used, although it is the same basic composition,
By mixing two kinds of lithium manganese composite oxides with slightly different compositions to form a positive electrode active material, it is possible to disperse the decomposition reaction of the electrolytic solution with heat generation, and the temperature rise due to a rapid chain reaction This can prevent the overcharged lithium secondary battery from calming before the temperature rise due to the chain reaction.

The present invention has been made based on the above findings, and provides a lithium secondary battery which does not significantly change battery characteristics such as a decrease in capacity and has excellent safety in overcharging. That is the task.

[0011]

Means for Solving the Problems To solve the above problems,
The lithium secondary battery of the present invention has a basic composition of LiMn.
_TwoO_FourOf spinel structure lithium manganese composite oxide
A lithium secondary battery as a positive electrode active material, wherein the positive electrode
The active material has the composition formula LiM_xMn_2-xO_Four(M is Li, M
g, at least one selected from Al, Ni, and Cu; 0
≦ x ≦ 0.1) and a composition formula L
iM_yMn_2-yO_FourThe second positive represented by (x <y ≦ 0.3)
And a polar active material.

That is, the lithium secondary battery of the present invention is a lithium secondary battery having a spinel structure lithium manganese composite oxide having the same basic composition. Battery. Although the detailed operation will be described later, by using two types of active materials having different compositions, it is possible to change the timing at which the decomposition of the electrolytic solution occurs in the overcharged state and to disperse the overcharge reaction in a timely manner. Then, the temperature rise due to the chain reaction can be made gentle, and in some cases, the overcharged lithium secondary battery can be calmed down before the temperature rise due to the chain reaction.

Therefore, it is possible to alleviate or avoid the deformation of the battery case due to the rapid oxidation reaction of the non-aqueous electrolyte caused when the temperature rises due to the chain reaction,
A lithium secondary battery with excellent overcharge safety. Also,
In the lithium secondary battery of the present invention, a substance that does not contribute to the charge / discharge reaction such as Li ₂ CO ₃ is not included in the positive electrode, and in that respect, the merit of the high energy density of the lithium secondary battery is hindered. Nor.

[0014]

In the following, a lithium manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ is used as a positive electrode active material.
The operation of the lithium secondary battery of the present invention, particularly the behavior at the time of overcharging, will be described by taking a lithium secondary battery using a carbon material as a negative electrode active material as an example.

The spinel structure has a composition formula L
In the case of a lithium manganese composite oxide represented by iMn ₂ O ₄ , O is arranged in a cubic close-packed structure as a basic structure, and Li is contained in 1/8 of the 4-coordinate position and M is contained in 1/2 of the 6-coordinate position.
n are regularly arranged. Since 32 Os are contained in the unit cell, the number of 4-coordinate positions is 64, which is twice as large as that of O, and 8 of 1/8 are Li, and the number of 6-coordinate positions is 32, and
Mn is arranged in 16 of the / 2. Therefore, those having a normal composition in which none of the constituent atoms are missing or excessive are (Li) _8a (Mn ₂ )
_16d (O ₄ ) _32e . 8a, 1
6d and 32e are respectively Wyckof in crystallography.
It means the position of f.

In the positive electrode of a lithium secondary battery, during charging, a reaction represented by the following equation, in which lithium is desorbed from lithium manganese composite oxide as a positive electrode active material, is performed.

(Li) _8a (Mn ₂ ) _16d (O ₄ ) _32e →
(□) _8a (Mn ₂ ) _16d (O ₄ ) _32e + Li ⁺ (□ means a site where an atom is missing) The lithium secondary battery of the above configuration has a normal battery voltage range in which stable charge and discharge can be repeated. As a general rule,
The discharge lower limit voltage is set to about 3 V and the charge upper limit voltage is set to about 4.2 V. As the charging proceeds, Li desorbs as the battery voltage increases.
i desorbs. Even after all Li is desorbed, charging exceeding the above charging upper limit voltage is performed, and when reaching a so-called overcharged state, the average valence of Mn becomes almost 4, the crystal structure becomes unstable, and finally O in the crystal is Leads to release. The released oxygen serves as a trigger, and decomposition of the non-aqueous electrolyte starts on the positive electrode surface. This is the start of the so-called overcharge reaction.

FIG. 1 shows a composition formula LiMn ₂ having a normal composition.
When a lithium secondary battery using a lithium manganese composite oxide represented by O ₄ as a positive electrode active material is continuously charged at a constant current to be in an overcharged state, an overcharge rate, a battery voltage, and a battery temperature are compared. The relationship is schematically shown. Here, the overcharge rate means the amount of electricity charged to the battery beyond the amount of electricity that can be charged in the normal battery voltage range (meaning the reference capacity) is one.

In FIG. 1, a battery voltage of 4.2 V is set as a normal charging upper limit voltage, and a case where the battery is charged to a voltage higher than the normal charging voltage is defined as overcharging. When the overcharge rate is 0 to about 0.05, the battery voltage continues to increase from 4.2 V to about 5.1 V. Then, when the overcharge rate exceeds about 0.05, the process moves to a region where the battery voltage does not significantly increase.
This region is a region where O is released from the lithium manganese composite oxide and the non-aqueous electrolyte starts to decompose. The decomposition reaction of the electrolytic solution is an exothermic reaction, and the battery temperature starts to rise due to the generated heat.

Since the decomposition of the non-aqueous electrolyte involves gas generation, the amount of the electrolyte decreases. This decrease in the electrolyte leads to suppression of the subsequent overcharge reaction. The internal pressure of the battery rises due to gas generation, but a safety valve is generally provided in a lithium secondary battery. When a predetermined pressure is reached, the safety valve opens to release the internal pressure, and when overcharging, Acts to ensure a certain degree of safety. In addition, lithium secondary batteries generally have a porous separator such as polyethylene or polypropylene interposed between the positive electrode and the negative electrode. As the temperature rises, this separator softens and closes, and further overcharge occurs. It acts to suppress the reaction. Further, depending on the installation state of the lithium secondary battery, if the heat radiation is good, the heating of the battery is suppressed. Under relatively mild overcharge conditions, such as a small charge current, due to the self-suppressing action of these lithium secondary batteries, the overcharged lithium secondary batteries calm down, the battery temperature gradually decreases,
The temperature rise due to the chain reaction is avoided.

However, under severe charging conditions such as a large charging current, the amount of heat generated per unit time is remarkable.
Even with the self-suppressing action, the rise in battery temperature cannot be suppressed. On the other hand, resistance heat generated by an increase in internal resistance due to phenomena such as a decrease in electrolyte solution and clogging of a separator outweighs heat dissipation and also causes a rise in battery temperature.
Such an increase in the temperature of the lithium secondary battery eventually causes an oxidation reaction of the electrolytic solution, and in some cases, adversely affects the lithium secondary battery such as deformation of the battery case. When the temperature rises suddenly and causes a chain reaction at a stretch, the deformation of the battery case becomes extremely remarkable, and avoiding the temperature rise due to such a rapid chain reaction can be avoided by using a lithium secondary battery. It is extremely effective for improving the safety of

In FIG. 1, when the overcharge rate exceeds 0.2, the battery voltage temporarily decreases slightly due to a decrease in battery resistance due to a rise in temperature. However, a sharp rise in the battery voltage immediately after that indicates an increase in internal resistance accompanied by blockage of the separator, and a change in the gradient of the temperature line indicates the start of temperature rise due to a chain reaction. The subsequent rapid rise of the temperature line indicates a rapid change in temperature leading to an oxidation reaction of the electrolytic solution. FIG. 1 is a schematic diagram showing a certain model, and some differences occur depending on the configuration of the lithium secondary battery, overcharge conditions, and the like. This is the same for other figures described later in the section of the present operation.

On the other hand, the lithium manganese composite oxide has a normal composition, and a lithium manganese composite oxide represented by Li _{1 + x} Mn _2-x O _{4 in} which a part of the Mn site is replaced by another element, for example, Li. Can also be easily synthesized. This is obtained by utilizing the position of the Wyckoff (Li) _8a (Li _x M
n _2-x ) _16d (O ₄ ) _32e . And
At the time of charging, a reaction represented by (Equation 1) in which lithium is desorbed from the lithium manganese composite oxide as the positive electrode active material is performed. Also in this case, as the normal battery voltage range in which stable charging and discharging can be repeated, the discharge lower limit voltage is set to about 3 V and the charging upper limit voltage is set to about 4.2 V.

(Li) _8a (Li _x Mn _2-x ) _16d (O ₄ ) _32e → (□ _1-3x Li _3x ) _8a (Li _x Mn _2-x ) _16d (O ₄ ) _32e + (1-3x) ) Li ⁺ (Formula 1) As can be seen from Formula 1, not all of Li is desorbed even at a battery voltage of 4.2 V, unlike the above-mentioned normal composition. That is, Li is not easily desorbed, and it is difficult to desorb all Li at the 8a position at a normal upper limit charging voltage. When the battery is further charged, a reaction represented by (Equation 2) is performed around a battery voltage of 5.05 V,
The remaining Li at the 8a position is eliminated.

(□ _1-3x Li _3x ) _8a (Li _x Mn _2-x ) _16d (O ₄ ) _32e → (□) _8a (Li _x Mn _2-x ) _16d (O ₄ ) _32e + 3 × Li ⁺ (formula 2) FIG. 2 shows the overcharge rate and the battery voltage when the lithium secondary battery using this Li _{1 + x} Mn _2-x O ₄ as the positive electrode active material was continuously charged at a constant current and brought into an overcharged state. Is schematically shown.

As shown in FIG. 2, when charging is continued, the battery voltage rises to around 5.05 V, and the overcharge rate becomes 0.05 to
The voltage is substantially constant up to around 0.3. In the region where the battery voltage is constant, Li, which does not desorb in the normal battery voltage range, moves to the negative electrode, and the reaction shown in (Equation 2) is performed. This reaction does not generate heat, so that the battery temperature hardly changes. When charging is further continued, the battery voltage rises to 5.1 V near the overcharge rate of 0.35, and becomes substantially constant up to about 0.5 of the overcharge rate. In this region where the voltage is constant, the non-aqueous electrolyte starts to decompose, and the battery temperature starts to rise due to the accompanying heat generation. Then, when the overcharge rate exceeds 0.5, the battery voltage temporarily decreases slightly due to a decrease in battery resistance due to an increase in temperature. However, immediately after that, the battery voltage sharply rises due to clogging of the separator and the like, and the battery temperature also sharply rises, indicating that the temperature has fallen due to a chain reaction.

That is, in a secondary battery using a lithium manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ and a normal composition as a positive electrode active material, all of Li is generally desorbed in a battery voltage range. Therefore, if the battery is further charged, the battery voltage immediately rises to a voltage at which the electrolytic solution is decomposed, and when the overcharge rate is around 0.05, the electrolytic solution is decomposed and the overcharge reaction is started. On the other hand, Mn
In a secondary battery in which a part of the site is replaced with Li as a positive electrode active material, all of Li is not desorbed even in a normal battery voltage range, and further charging is continued to increase the battery voltage to 5.0.
When the voltage reaches about 5 V, the elimination reaction of the remaining Li at the 8a position is performed. During that time, the battery voltage is kept constant and no decomposition of the electrolyte occurs. Then, after all of the Li at the position 8a is desorbed, the battery voltage reaches a voltage at which the electrolytic solution is decomposed, the electrolytic solution is decomposed, and an overcharge reaction is started. That is, M
The ease of desorption of Li during charging changes depending on the substitution ratio of the n-site with another element, and the start time of the overcharge reaction differs between the two.

Therefore, in summary, even when the lithium manganese composite oxides have the same basic composition, the timing of causing the overcharge reaction differs due to a slight difference in the composition, particularly the difference in the content of Mn. The lithium secondary battery of the present invention in which the mixed mixture is used as the positive electrode active material is a lithium secondary battery that has a small possibility of causing a temperature rise due to a chain reaction by performing the overcharge reaction in a dispersed manner. Also,
Even when the temperature rises due to the chain reaction, it progresses relatively slowly, so that the effect on the lithium secondary battery such as deformation of the battery case is small.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of a lithium secondary battery of the present invention will be described in detail by dividing into items such as a positive electrode active material, a configuration of a positive electrode, and an overall configuration of a lithium secondary battery.

<Positive Electrode Active Material> (1) Target Lithium-Manganese Composite Oxide The lithium secondary battery of the present invention comprises, as a positive electrode active material, a spinel-structured lithium-manganese composite oxide having a basic composition of LiMn ₂ O _4. adopt. “Let the basic composition be LiMn ₂ O ₄ ” means not only that of the normal composition but also that of L in the crystal structure.
This means that i and Mn sites include those in which a part of the site is replaced by another element. Also, not only those of stoichiometric composition,
Inevitable constituent element deficiency, excess presence,
Alternatively, it means that a mixture of other impurities is allowed. The same applies to the first positive electrode active material and the second positive electrode active material described below.

(2) First Positive Electrode Active Material The first positive electrode active material, which is one of the positive electrode active materials constituting the lithium secondary battery of the present invention, has a composition formula of LiM _x Mn _2-x O
₄ (M is at least one selected from the group consisting of Li, Mg, Al, Ni, and Cu; 0 ≦ x ≦ 0.1). The first positive electrode active material has a smaller substitution ratio of Mn sites by M in the crystal structure than the second positive electrode active material. Specifically, a normal composition LiMn ₂ O ₄ in which a part of the Mn site is not replaced with M
(In the case of x = 0), and a composition formula LiM _x Mn _2-x O in which a part of the Mn site is slightly substituted with M
₄ Including those represented by (0 <x ≦ 0.1).

Composition formula LiM _x in which Mn site is substituted by M
When the one represented by Mn _2-x O ₄ is adopted, the substitution ratio of M, that is, the value of x in the composition formula, is set to 0 <x ≦ 0.1. In the first positive electrode active material having a small substitution ratio with M,
When x exceeds 0.1, the substitution ratio of the second positive electrode active material described later may be larger than that, and in the normal battery voltage range, L that is inserted and extracted with charge and discharge is L.
This is because the amount of i decreases too much, and the capacity of the lithium secondary battery decreases significantly.

The metal element M is higher than the average valence of Mn ions.
Li, Mg, Al, Ni, Cu
At least one selected from the group consisting of: In particular, low valence
Is it possible to obtain the effect of replacing even a small amount of ions?
Therefore, it is desirable to use Li. Therefore, the first positive
As a polar active material, a composition formula Li substituted by Li_{1 + x}Mn_Two
-xO_FourIt is desirable to adopt the one represented by

In the above composition range, there are various lithium manganese composite oxides having slightly different compositions. The lithium secondary battery of the present invention does not prevent mixing of a plurality of types of lithium manganese composite oxides having slightly different compositions into the first positive electrode active material.

(3) Second positive electrode active material The second positive electrode active material, which is another positive electrode active material constituting the lithium secondary battery of the present invention, has a composition formula of LiM _y Mn _2-y O
_{4 This} is a lithium-manganese composite oxide represented by (x <y ≦ 0.3). The second positive electrode active material has a larger substitution ratio of Mn sites by M in the crystal structure than the first positive electrode active material.

As described above, the metal element M is Li, Mg, A
1, at least one selected from Ni, Cu, and Mn
When considering that the valence of the ion can be increased by a small amount of substitution, the composition formula Li _{1 + y} Mn _2-y substituted by Li
It is desirable to adopt the one represented by O ₄ .

The substitution ratio of M at the Mn site, that is, the value of y in the composition formula, is set to x <y ≦ 0.3. This is because, as in the case of the first positive electrode active material, when y exceeds 0.3, the amount of Li absorbed and desorbed during charging and discharging in a normal battery voltage range becomes too small, and lithium secondary This is because the reduction in battery capacity becomes significant. In particular, when considering the capacity of the battery, it is desirable that y ≦ 0.2.

When the value of y is close to the value of x, M
Can not make much difference in the replacement ratio in
The dispersion effect of the start time of the overcharge reaction is reduced. Therefore, in order to further disperse the start time of the overcharge reaction, the difference between the replacement ratio of the first active material and the replacement ratio of the second active material is set to 0.1.
It is desirable that the number be 05 or more. Considering this point, x
It is desirable that + 0.05 ≦ y.

As in the case of the first active material, various lithium manganese composite oxides having slightly different compositions exist within the above composition range. In the lithium secondary battery of the present invention, it does not prevent the mixing of a plurality of types of lithium manganese composite oxides having slightly different compositions into the second positive electrode active material.

(4) Method for Producing Positive Electrode Active Material The various lithium manganese composite oxides used as the positive electrode active material in the lithium secondary battery of the present invention are not particularly limited in their production methods. Various known production methods such as a solid-phase reaction method, a precipitation method from an aqueous solution, and a spray combustion method can be employed to produce the lithium-manganese composite oxide.

For example, when a lithium manganese composite oxide represented by LiMn ₂ O ₄ or represented by the composition formula Li _{1 + y} Mn _2-y O ₄ is produced by a solid-state reaction method,
A lithium compound serving as a source and a manganese compound serving as a Mn source are mixed in accordance with the composition ratio of the constituent elements of the lithium-manganese composite oxide to be synthesized.
At a temperature of 0 to 900 ° C. in an oxygen atmosphere or air,
It can be synthesized by firing for 8 to 24 hours. On this occasion,
As the lithium compound serving as the Li source, lithium hydroxide, lithium carbonate or the like can be used, and as the manganese compound serving as the Mn source, manganese dioxide or the like can be used.

(5) Constituent Ratio of First Positive Electrode Active Material and Second Positive Electrode Active Material The first positive electrode constituting the positive electrode active material of the lithium secondary battery of the present invention.
The composition ratio of the positive electrode active material and the second positive electrode active material is not particularly limited, and may be a composition ratio necessary for sufficiently achieving the effect of dispersing and performing the overcharge reaction.

For example, a composition formula Li _{1 + x} M where M is Li
a first positive electrode active material represented by n _2-x O ₄ , and a composition formula Li _{1 + y}
The composition ratio of the first positive electrode active material to the second positive electrode active material represented by Mn _2-y O ₄ is preferably more than 10% by weight and less than 60% by weight when the total of both is 100% by weight. . This preferred range has been confirmed by experiments. When the amount of any of the positive electrode active materials is smaller than this range, the effect due to the dispersion of the overcharge reaction is smaller than that in the range, and the larger amount is preferable. This is because the reaction is governed by the overcharge reaction, and the possibility of a temperature rise due to a rapid chain reaction increases. Further, according to verification by experiment, when the first positive electrode active material has a composition ratio of 20 wt% or more and 50 wt% or less, a lithium secondary battery with more secured overcharge safety such as a small deformation of the battery case is obtained. Become. Furthermore, in order to obtain a lithium secondary battery which is very unlikely to cause a temperature rise due to a chain reaction and is more excellent in safety, the first positive electrode active material must be 20 wt% or more and 30 wt% or more.
% Has been confirmed to be more desirable.

<Structure of Positive Electrode> The structure of the positive electrode of the lithium secondary battery of the present invention is not particularly limited except for the above-described positive electrode active material. Good. The above-described positive electrode active material is produced by preparing each of the lithium-manganese composite oxides serving as the first positive electrode active material and the second positive electrode active material as a powder, and forming a powder mixture obtained by sufficiently mixing the powder. The powder mixture can be formed by binding the powder mixture with a binder. More specifically, first, a lithium manganese composite oxide as a positive electrode active material, a conductive material, and a binder are mixed, and a solvent for dispersing these is added. Prepare the ingredients. Next, this positive electrode mixture may be applied to the surface of a positive electrode current collector such as an aluminum foil by a coating machine or the like, and dried to form a positive electrode mixture containing only solids in a layered form.
Thereafter, if necessary, the active material density may be increased by performing compression using a compressor such as a roll press. The positive electrode in this form is in a sheet shape, and may be cut into a size suitable for a battery to be manufactured and used.

The conductive material is for ensuring the electrical conductivity of the positive electrode, and one or a mixture of two or more powdered carbon materials such as carbon black, acetylene black and graphite is used. be able to. The binding agent plays a role of binding the active material particles and the conductive material particles, and may be a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. . Further, as a solvent for dispersing, N-methyl-2-
Organic solvents such as pyrrolidone can be used. In addition,
The mixing ratio of the active material, the conductive material, and the binder (only solid content) in the positive electrode mixture is 2 to 20 parts by weight of the conductive material, 100 parts by weight of the positive electrode active material, and the positive electrode binder in weight ratio. The amount may be 1 to 20 parts by weight, and the amount of the solvent added may be an appropriate amount according to the characteristics of the coating machine or the like.

<Overall Configuration of Lithium Secondary Battery> The configuration of the lithium secondary battery of the present invention is not particularly limited, except for the above-described positive electrode, and may follow the configuration of a known lithium secondary battery.

The negative electrode opposed to the positive electrode is formed by sheeting metal lithium, a lithium alloy, or the like, or by pressing the sheet into a current collector net made of nickel, stainless steel, or the like. Is also good. However, in consideration of dendrite precipitation and the like, a negative electrode using a carbon material capable of inserting and extracting lithium as an active material can be used in order to obtain a lithium secondary battery having excellent safety. Examples of the carbon substance that can be used include natural or artificial graphite, fired organic compounds such as phenolic resins, and powders such as coke. In this case, a binder is mixed with the negative electrode active material,
A negative electrode mixture formed into a paste by adding an appropriate solvent is applied to the surface of a metal foil current collector made of copper or the like, followed by drying to form the mixture. In addition,
When the carbon material is used as the negative electrode active material, a fluorine-containing resin such as polyvinylidene fluoride or the like can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent, similarly to the positive electrode.

The lithium secondary battery of the present invention comprises, similarly to a general lithium secondary battery, a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte, and the like. . The separator separates the positive electrode and the negative electrode and holds the electrolyte, and a thin microporous film of polyethylene, polypropylene, or the like can be used. It is desirable that the separator softens and closes its pores when the battery temperature rises to some extent, so that the separator sufficiently exhibits a so-called shutdown effect of stopping the battery reaction.

The non-aqueous electrolyte is an electrolyte in an organic solvent.
The lithium salt is dissolved, and as the organic solvent,
Aprotic organic solvents such as ethylene carbonate,
Propylene carbonate, dimethyl carbonate, die
Chill carbonate, ethyl methyl carbonate, γ-butyl
Tyrolactone, acetonitrile, 1,2-dimethoxye
Tan, tetrahydrofuran, dioxolan, methyle chloride
Use one or a mixture of two or more of these
be able to. The electrolyte to be dissolved is Li
I, LiClO_Four, LiAsF₆, LiBF_Four, LiP
F₆, LiN (CF _ThreeSO_Two)_TwoUse lithium salt such as
Can be.

The lithium secondary battery of the present invention configured as described above has various shapes such as a cylindrical type and a laminated type. Regardless of the shape used, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a current collecting lead extends from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal that lead to the outside. The electrode body is inserted into the battery case together with the non-aqueous electrolyte, and the battery case is sealed to complete the battery.

Since the lithium secondary battery of the present invention aims to enhance safety during overcharge, a safety valve for releasing the internal pressure of the battery when the internal pressure of the battery reaches a predetermined pressure is provided. It is desirable to provide it in the battery case. In addition, PTC elements, temperature fuses, and other means that detect a rise in the temperature of the secondary battery and limit the current when the temperature reaches a predetermined temperature, etc., are used in combination, to ensure safety during overcharge. Can be secured more sufficiently.

<Allowance of Other Embodiments> Although the embodiment of the lithium secondary battery of the present invention has been described above, the above-described embodiment is merely an embodiment, and the lithium secondary battery of the present invention has the above-described configuration. The present invention can be implemented in various forms including various modifications and improvements based on the knowledge of those skilled in the art, including the embodiment.

[0053]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, based on the above embodiment, a predetermined first
A lithium secondary battery of the present invention constituted by mixing a positive electrode active material and a second positive electrode active material was produced. For comparison, a lithium secondary battery using a predetermined first positive electrode active material or a predetermined second positive electrode active material alone was also manufactured. An overcharge test was performed on the manufactured lithium secondary batteries, and high safety of the lithium secondary batteries of the present invention was confirmed. Hereinafter, these will be described in detail.

<Prepared Lithium Secondary Battery> The structure of the prepared lithium secondary battery will be described with reference to FIG. The manufactured lithium secondary battery is a lithium secondary battery in which a roll-shaped electrode body in which electrodes are wound is inserted into a cylindrical battery case. FIG. 3 shows the cross section.

The present lithium secondary battery 1 includes an electrode body 10,
Battery case 2 for sealing electrode body 10 together with non-aqueous electrolyte
0, a positive electrode terminal 30 and a negative electrode terminal 40 attached to the battery case 20 and electrically connected to the electrode body 10.
The electrode body 10 includes a sheet-shaped positive electrode 11 and a sheet-shaped negative electrode 1.
2 is a roll having a separator 13 sandwiched therebetween and wound around a winding core 14.

The positive electrode 11 is prepared by mixing 5 parts by weight of graphite as a conductive material and 5 parts by weight of polyvinylidene fluoride as a binder with 90 parts by weight of a positive electrode active material (details will be described later). Of N-methyl-2-pyrrolidone, and kneading them sufficiently to prepare a paste-like positive electrode mixture, and then applying this positive electrode mixture to both sides of a 15 μm-thick aluminum foil current collector. , Dried, and pressed to increase the density of the positive electrode mixture. The positive electrode 11 has a thickness of 75 μm and a size of 1 μm.
It was 24 mm x 3100 mm.

The negative electrode 12 employs spherical artificial graphite as the negative electrode active material. First, 95 parts by weight of this graphite is mixed with 5 parts by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N- Methyl-2-pyrrolidone was added, and these were sufficiently kneaded to prepare a paste-like negative electrode mixture. Then, the negative electrode mixture was applied to both surfaces of a 10 μm-thick copper foil current collector, and dried. And pressed to increase the density of the negative electrode mixture. The negative electrode 12 has a thickness of 83 μm.
m, and its size was 130 mm × 3200 mm.

The separator 13 was made of a porous polypropylene sheet, had a thickness of 25 μm, a size of 136 mm × 3400 mm, and used two sheets thereof. The winding core 14 includes an aluminum winding core 14 a made of an aluminum alloy located on the positive electrode terminal side, and an aluminum winding core 14.
and a resin-wound core portion 14b made of resin and coaxially screw-connected to a.

The electrode body 10 produced by winding the positive electrode 11 and the negative electrode 12 on the wound core 14 with the separator 13 interposed therebetween has a positive electrode lead 11a on the positive electrode 11 and the negative electrode 12 at the time of winding. And a negative electrode lead 12a, and the leads 11a, 12a
From each wound end.

The battery case 20 includes a cylindrical outer can 21 made of an aluminum alloy, and a disc-shaped positive electrode side cover plate 22 and a negative electrode side cover plate 23 made of an aluminum alloy joined to both open ends of the outer case 21, respectively. Consists of The outer can 21 has a thickness of 1.5 mm, an outer diameter of 33 mm, and a length of 1 mm.
70 mm. The positive electrode side cover plate 22 and the negative electrode side cover plate 23
The plate thickness is 1.5 mm, and it has a disk shape with an outer diameter substantially equal to the inner diameter of the outer can 21. The outer can 21 and the positive-side lid plate 22 and the negative-side lid plate 23 are joined together by laser welding at a point A in the figure.

Each of the positive cover plate 22 and the negative cover plate 23 is provided with a safety valve 24 which opens when the internal pressure of the battery case 20 exceeds a predetermined pressure (the positive electrode is not shown). Further, the negative electrode side cover plate 23 is further provided with an electrolyte injection port 25, and an injection hole plug 26 for sealing the electrolyte injection port 25 is screwed and attached. The safety valve is designed to open when the internal pressure of the battery becomes 15 MPa.

The positive electrode terminal 30 is made of an aluminum alloy.
The current collecting unit 30a includes a current collecting unit 30a and a bolt-shaped external terminal unit 30b.
The external terminal 30b is screwed to the positive terminal mounting hole 22a formed in the positive cover plate 22 of the battery case 20 with its tip protruding outside the battery. 32 and a nut 33, which are insulated from the battery case 20. A strip-shaped positive electrode lead 11a extending from the electrode body 10 is joined to the periphery of the current collector 30a, and the positive electrode terminal 30 and the positive electrode 1 are connected to each other.
1 and electrical continuity are secured.

The negative electrode terminal 40 is made of a copper alloy.
a, and a bolt-shaped external terminal portion 40b. The current collecting portion 40a is screwed and connected to the resin wound core portion 14b of the wound core 14, and the external terminal portion 40b has a tip outside the battery. In a protruding state, a gasket 41 is inserted through a gasket 41 into a negative electrode terminal mounting hole 23 a provided in the negative electrode side lid plate 23 of the battery case 20.
The battery case 20 is provided with a washer 42 and a nut 43 to keep the battery case 20 insulated. A strip-shaped negative electrode lead 12a extending from the electrode body 10 is joined to the periphery of the current collector 40a to ensure electrical conduction between the negative electrode terminal 40 and the negative electrode 12.

As the non-aqueous electrolyte to be injected, a solution prepared by dissolving LiPF ₆ at a concentration of 1 M in a mixed organic solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 was used. The injection amount of the non-aqueous electrolyte was 70 cc.

As the lithium secondary batteries, eight kinds of lithium secondary batteries differing only in the constitution of the positive electrode active material were produced. The first positive electrode active material constituting the positive electrode active material is a lithium nickel composite oxide having a regular composition represented by a composition formula LiMn ₂ O ₄ , and the second positive electrode active material is a composition formula Li _1.1 Mn _1.9 O _4. The lithium manganese composite oxide represented by each was used. The positive electrode active material of each lithium secondary battery includes a first positive electrode active material and a second positive electrode active material.
The composition ratio with the positive electrode active material is different. The manufactured lithium secondary batteries are numbered # 1 to # 8, and the composition ratio of the first positive electrode active material and the second positive electrode active material in each lithium secondary battery is shown in Table 1 below.

[0066]

[Table 1]

<Overcharge test> Lithium of # 1 to # 8
Standard condition for rechargeable batteries that also serves as conditioning
Cycle charge / discharge for the amount measurement was performed. This cycle
Charge and discharge were performed at an environmental temperature of 25 ° C. First time
Cycle is a current density of 0.25 mA / cm ^TwoConstant current
To charge the battery voltage to 4.2V, and then charge the battery
Charge with constant voltage (total charge time 6 hours), then
And the current density is 0.2 mA / cm^TwoBattery voltage at constant current
Discharge was performed up to 3.0 V. Furthermore, the second time
From the first cycle to the fifth cycle, the current density is 1 mA
/ Cm^TwoTo a battery voltage of 4.2V with a constant current of
Furthermore, constant voltage charging is performed at the battery voltage (total charging time
2 hours), then a current density of 1 mA / cm^TwoAt a constant current of
Repeat the cycle of discharging to battery voltage 3.0V
Was. Then, with the discharge capacity of the fifth cycle,
Is the reference capacity of each lithium secondary battery.
Was. As a result of this cycle charge and discharge,
The capacity showed substantially the same value. Also, any lithium
The discharge capacity of the secondary battery between the second and fifth cycles
Since there was almost no difference in the amount, these lithium
The normal operating voltage range of the secondary battery is 3.0 to 4.2 V
Was confirmed to be appropriate.

Next, an overcharge test was performed on each lithium secondary battery. The conditions of the overcharge test were as follows. First, assuming that the current value when discharging the reference capacity of each battery in one hour (current value in one hour rate discharge) is 1 C, the battery voltage is 4.2 V with a current corresponding to 1 C.
The battery was charged at a constant current until the battery voltage reached, and the battery was charged at a constant voltage (total charging time 3 hours). Next, a thermocouple for measuring the battery temperature was attached to the surface of the battery case, and charging was performed with a current corresponding to 10 C until the upper limit voltage reached 22 V.

The safety of the battery in the overcharge test was evaluated by visual observation of the opening state of the safety valve during the overcharge process, the presence or absence of the oxidation reaction of the electrolyte, and the degree of deformation of the battery case. The overall evaluation was made based on the results. The evaluation results are shown in Table 2 below.

[0070]

[Table 2]

As is clear from Table 2, the safety valves are open for all lithium secondary batteries. # 1 and # 8 using the first positive electrode active material or the second positive electrode active material alone
In the lithium secondary battery, the battery case was significantly deformed, and it was found that there was a problem in safety. In contrast, the first
# 2 to # using a mixture of the positive electrode active material and the second positive electrode active material
The lithium secondary battery of No. 7 is slightly different from the lithium secondary batteries of # 2 and # 7 having a bias in the composition ratio of the positive electrode active material, but other lithium secondary batteries of # 3 to # 6 No deformation of the battery case was observed. Furthermore, it can be seen that the lithium secondary batteries of # 3 and # 4 do not cause an oxidation reaction of the electrolyte, and are extremely safe for overcharge.

Next, the overcharge rate and the battery voltage of the lithium secondary battery # 8 using only the first positive electrode active material and the lithium secondary batteries # 3 and # 4 in which even the oxidation reaction of the electrolyte was not observed. Is shown in FIG. 4 and the relationship between the overcharge rate and the battery temperature is shown in FIG.

As is apparent from FIGS. 4 and 5, # 8
In the lithium secondary battery described above, the overcharge reaction starts slowly, and the temperature rises at once due to a chain reaction, which causes a significant deformation of the battery case. In contrast, #
In the lithium secondary batteries of Nos. 3 and # 4, a gradual overcharge reaction starts from the initial stage of overcharge, and calms before the temperature rise due to the chain reaction due to the self-suppressing action of the lithium secondary battery described above. It turns out that there is. Therefore,
It can be seen that this is a highly safe lithium secondary battery that does not cause an oxidation reaction of the electrolyte.

When the above results are combined, the basic composition can be expressed as Li
In a lithium secondary battery using a lithium manganese composite oxide as Mn ₂ O ₄ as a positive electrode active material, the positive electrode active material is formed by mixing the first positive electrode active material and the second positive electrode active material described above. Thereby, it can be confirmed that the overcharge reaction occurs in a timely manner and the temperature rise due to a rapid chain reaction accompanied by a significant deformation of the battery case can be avoided. Further, by making the composition ratio of the first positive electrode active material and the second positive electrode active material appropriate, the overcharged lithium secondary battery can be calmed down before the temperature rises due to the chain reaction, It can be confirmed that the lithium secondary battery has extremely excellent overcharge safety.

[0075]

According to the present invention, there is provided a lithium secondary battery using a lithium manganese composite oxide having a basic composition of LiMn ₂ O ₄ as a positive electrode active material.
The first positive electrode active material represented by M _x Mn _{2 -x} O ₄ and the second positive electrode active material represented by the composition formula LiM _y Mn _{2 -y} O ₄ are included. In the lithium secondary battery of the present invention having such a configuration, since the decomposition time of the electrolyte, which is the start of the overcharge reaction, is different, the overcharge reaction is dispersed in time, and the temperature rise due to a rapid chain reaction is avoided. Alternatively, a lithium secondary battery excellent in overcharge safety, in which an overcharged state can be calmed down before temperature rise due to a chain reaction.

[Brief description of the drawings]

FIG. 1 shows a case where a lithium secondary battery using a lithium manganese composite oxide represented by a composition formula LiMn ₂ O _{4 as} a normal composition as a positive electrode active material is continuously charged with a constant current to be in an overcharged state. The relationship between the overcharge rate, the battery voltage, and the battery temperature is schematically shown.

FIG. 2 A lithium secondary battery using a lithium manganese composite oxide represented by the composition formula Li _{1 + x} Mn _2-x O ₄ as a positive electrode active material is continuously charged at a constant current to be in an overcharged state. The relationship between the overcharge rate and the battery voltage at this time is schematically shown.

FIG. 3 shows a structure of a lithium secondary battery manufactured for use in an overcharge test.

FIG. 4 shows the relationship between the overcharge rate and the battery voltage of the lithium secondary battery of # 8 and the lithium secondary batteries of # 3 and # 4 in Examples.

FIG. 5 shows the relationship between the overcharge rate and the battery temperature of the lithium secondary batteries of # 8 and # 3 and # 4 in Examples.

[Explanation of symbols]

 1: cylindrical lithium secondary battery 10: electrode body 20: battery case 21: outer can 22: positive electrode side cover plate 23: negative electrode side cover plate 30: positive electrode terminal 40: negative electrode terminal

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Jiro Mizuno 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside of Toyota Central Research Laboratory Co., Ltd. (72) Inventor Fumi Miura Nagakute-machi, Aichi-gun, Aichi No. 41, Chochu-Yokomichi, Toyota Central Research Laboratory Co., Ltd. (72) Inventor Tatsumi Hioki 41, Nagakute-cho, Aichi-gun, Aichi, Japan No. 1 Toyota-cho, Toyota City, Aichi Prefecture F-term (reference) in Toyota Motor Corporation 5H029 AJ12 AK03 AK19 AL06 AL07 AL08 AL12 AM02 AM03 AM05 AM07 BJ02 BJ14 HJ01 HJ02 5H050 AA15 BA16 BA17 CA09 CB07 CB08 CB09 CB12 FA05 HA01 HA02

Claims (3)

[Claims] 1. A lithium secondary battery using a lithium manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ as a positive electrode active material, wherein the positive electrode active material has a composition formula: LiM _x Mn _2-x O ₄ (M is Li, Mg, Al, N
i, at least one selected from Cu; 0 ≦ x ≦ 0.
A first positive electrode active material represented by 1) and a second positive electrode active material represented by a composition formula: LiM _y Mn _2-y O ₄ (x <y ≦ 0.3) Lithium secondary battery. 2. In the above composition formula, M is Li, and y is
2. The lithium secondary battery according to claim 1, wherein the range of x + 0.05 ≦ y ≦ 0.2. 3. The composition ratio of the first positive electrode active material and the second positive electrode active material in the positive electrode active material is 10 wt% when the total of both is 100 wt%. 3. The lithium secondary battery according to claim 2, wherein the content is more than 60 wt%.