JP2002141063A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery excellent in safety when overcharged without affecting greatly on battery characteristics such as capacity deterioration. SOLUTION: With the lithium secondary battery with regular-array sodium chloride structure lithium transition metal compound oxide having a basic composition of LiMO2 (M is at least a kind selected from Ni, Co, and Mn) as a positive electrode active material, the positive electrode active material is structured with inclusion of a first and a second positive electrode active materials with different compositions. The first positive electrode active material is expressed with a composition formula of Li1-xM1+xO2(0<=x<=0.1), and the second positive electrode active material is expressed with a composition formula of Li1+yM'y2 M1-yO2 (where, M' is at least a kind selected from Mg, Cu, and Zn; y1+y2=y; 0<=y1<=0.1; 0<=y2<=0.1; 0<y<=0.1). Overcharge reaction can be dispersed in time, which makes it possible to slacken temperature rise due to chain reaction or to suppress the temperature rise due to the chain reaction.

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 phenomenon of insertion and extraction 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.

However, a lithium secondary battery uses a non-aqueous electrolytic solution using an organic solvent as an electrolytic solution as a constituent element, and therefore requires sufficient consideration for its safety. In particular, when the battery is overcharged, it causes a phenomenon such as an increase in internal pressure due to decomposition of the electrolyte due to the overcharge reaction, and furthermore, a temperature rise due to a chain of exothermic reactions. Therefore, a lithium secondary battery having a high level of demand has been eagerly desired.

As a technique for improving the safety of a lithium secondary battery, it is a well-known technique to install a safety valve in a battery case. 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.

[0005] It is known that in the initial stage of the overcharge state, the electrolytic solution 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.

[0007]

The present inventor has studied the overcharge reaction of a lithium secondary battery, particularly the overcharge reaction of a positive electrode active material, and has gained some knowledge through many experiments. Although a lithium secondary battery generally uses a lithium transition metal composite oxide as a positive electrode active material, even a lithium transition metal composite oxide having the same basic composition,
By changing the composition, the battery voltage at which the overcharge reaction occurs can be varied. By utilizing this phenomenon, by mixing two kinds of lithium transition metal composite oxides having the same basic composition but slightly different compositions to form a positive electrode active material, the overcharge reaction can be dispersed and performed. Thus, it is possible to avoid the temperature rise due to a rapid chain reaction or to calm down the overcharged lithium secondary battery 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 is excellent in safety at the time of overcharging. That is the task.

[0009]

Means for Solving the Problems To solve the above problems,
Therefore, the lithium secondary battery of the present invention has a basic composition of LiMO
_Two(M is at least one selected from Ni, Co, and Mn.
Species) Ordered Layered Rock Salt Structure Lithium Transition Metal Composite
A lithium secondary battery using an oxide as a positive electrode active material,
The positive electrode active material has a normal composition or an excessive amount of M.
Formula Li_{1- x}M_{1 + x}O_Two(0 ≦ x ≦ 0.1)
A first positive electrode active material, and a composition formula Li deficient in M.
_{1 + y1}M '_y2M_1-yO_Two(M 'is selected from Mg, Cu, Zn
Y1 + y2 = y; 0 ≦ y1 ≦ 0.1;
0 ≦ y2 ≦ 0.1; 0 <y ≦ 0.1)
And a polar active material.

That is, the lithium secondary battery of the present invention is an ordered layered rock salt structure lithium transition metal composite oxide having the same basic composition, and two kinds of slightly different compositions are mixed to form a positive electrode active material. Lithium secondary battery.
Although the detailed operation will be described later, the difference in the composition causes a difference in the battery voltage that causes the overcharge reaction of each of the lithium composite oxides serving as the first and second positive electrode active materials. However, the temperature rise due to the chain reaction can be made gradual, and in some cases, the overcharged lithium secondary battery can be calmed down before the temperature rise due to the rapid chain reaction.

Therefore, it is possible to alleviate or avoid deformation of the battery case due to a rapid oxidation reaction of the non-aqueous electrolyte which occurs 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.

[0012]

The lithium secondary battery according to the present invention will now be described, taking as an example a lithium secondary battery in which a lithium nickel composite oxide having an ordered layered rock salt structure having a basic composition of LiNiO ₂ as a positive electrode active material and a carbon material as a negative electrode active material. The operation of the secondary battery, in particular, the behavior during overcharge will be described.

The ordered layered rock salt structure has a hexagonal crystal structure. In the case of a lithium nickel composite oxide having a basic composition of LiNiO ₂ , an Li layer mainly composed of Li, mainly N
There is a Ni layer made of i and two O layers made of O, and each layer is repeatedly laminated in the order of a Li layer, an O layer, a Ni layer, and an O layer. The unit lattice in the crystal structure is composed of three repetitions, that is, 12 layers. Therefore, those having a normal composition in which none of the constituent atoms are missing or excessive are (Li) _3a (Ni) _3b (O ₂ ) _6c
It can be expressed by the formula In the formulas, 3a, 3b, and 6c respectively indicate positions of Wyckoff in crystallography.

In the positive electrode of a lithium secondary battery, at the time of charging, a reaction represented by the following formula, in which lithium is released from lithium nickel composite oxide, which is a positive electrode active material, is performed.

(Li) _3a (Ni) _3b (O ₂ ) _6c →
(□ _m Li _1-m ) _3a (Ni) _3b (O ₂ ) _6c + mLi ⁺ (□
Means a site where an atom is lost.) The lithium secondary battery having the above configuration generally has a normal battery voltage range in which stable charge and discharge can be repeated.
The discharge lower limit voltage is set to about 3 V, and the charge upper limit voltage is set to about 4.1 V. With the progress of charging, Li desorbs as the battery voltage rises, and when the battery voltage is 4.1 V, m = 0.
Li is released up to about 6 to 0.7. In the case of overcharging, that is, charging exceeding the charging upper limit voltage, Li is further desorbed, leading to a state in which almost no Li is present in the lithium nickel composite oxide. In this state, the average valence of Ni almost approaches 4, and the crystal structure becomes unstable, eventually leading to the release of O in the crystal. 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 LiNiO having a normal composition.
The relationship between the overcharge rate, the battery voltage, and the battery temperature when the lithium secondary battery using the lithium-nickel composite oxide represented by ₂ as the positive electrode active material is continuously charged at a constant current and brought into an overcharged state. Are shown schematically. 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 4.1 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. At an overcharge rate of 0 to about 0.15, the battery voltage continues to increase from 4.1V to about 4.7V. This region is defined as Li expressed by the above reaction formula.
Is the region where desorption continues. When the overcharge rate exceeds about 1.5, the operation shifts to a region where the battery voltage does not significantly increase. This region is a region where O is released from the lithium nickel 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, a sudden change in the battery voltage at a point where the overcharge rate exceeds 0.4 indicates an increase in the internal resistance accompanying blockage of the separator, and a change in the gradient of the temperature line. It shows the onset of temperature rise due to the 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.

The lithium nickel composite oxide has a regular composition
In addition, the composition formula Li in which Ni is missing_{1 + y}Ni_1-yO_Twoso
Those represented can also be easily synthesized. This one
Adopts the above Wyckoff position to obtain (Li)_3a
(Li_yNi_1-y)_3b(O_Two)_{6 c}It can be expressed by the formula
In other words, a crystal structure in which a part of the Ni site is replaced by Li
With. FIG. 2 shows a lithium secondary battery using this as a positive electrode active material.
When the secondary battery continues to be charged with a certain current and becomes overcharged
The relationship between the overcharge rate and the battery voltage at
This is schematically shown together with the case of the above.

As shown in FIG. 2, a lithium secondary battery using a lithium nickel composite oxide having a regular composition as a positive electrode active material causes an overcharge reaction at about 4.7 V, while a composition formula Li _{1 In} a lithium secondary battery using a lithium nickel composite oxide (Ni deficient type) represented by _{+ y} Ni _1-y O ₂ (y ≒ 0.05) as a positive electrode active material, an overcharge reaction occurs at about 5.0 V. Is generated. The difference in the battery voltage at which this overcharge reaction occurs is that the normal composition has an average valence of Ni of 3 before lithium desorption, whereas the Ni deficiency battery has an average valence of 3 or more. Is due.

Here, the behavior in overcharging of a lithium secondary battery using a lithium nickel composite oxide mixed with a normal composition and a Ni deficient type as a positive electrode active material will be considered. FIG. 3 shows a constant current of a lithium secondary battery using, as a positive electrode active material, a mixture obtained by mixing a lithium-nickel composite oxide having a regular composition and the above-described Ni-deficient lithium-nickel composite oxide at a weight ratio of 1: 1. 4 schematically shows the relationship between the overcharge rate and the battery voltage when the battery is kept in the overcharged state. Note that, in order to facilitate understanding, a voltage increase due to softening and closing of the separator is omitted in this figure.

As shown in FIG. 3, in the case of a mixture in which the two are mixed at a ratio of 1: 1, first, the overcharge rate is about 0.08 due to the difference in the overcharge reaction starting potential of each lithium nickel composite oxide. When the battery voltage reaches about 4.7 V, the overcharge reaction due to the normal composition starts. Then, charging proceeds at a substantially constant voltage, the battery voltage rises again at an overcharge rate of about 0.23, and an overcharge reaction due to the Ni-deficient type starts at an overcharge rate of about 0.25. . Thus, in a lithium secondary battery using a mixture of both, the overcharge reaction occurs in a dispersed manner. Dispersion of the overcharge reaction reduces the calorific value per unit time from the start of overcharge, suppresses a sharp rise in temperature when falling into overcharge, and greatly reduces the possibility of a rise in temperature due to a chain reaction. It can be.

Therefore, in summary, even when the lithium-nickel composite oxides have the same basic composition, the timing of causing the overcharge reaction differs due to a slight difference in the composition, in particular, a difference in the content of Ni. 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 a temperature rise due to a chain reaction occurs, it proceeds relatively slowly, so that the influence of the deformation of the battery case on the lithium secondary battery is small.

The lithium-nickel composite oxide has a normal composition and a Ni-deficient composition, and also includes, for example, an excess of Ni represented by the composition formula Li _1-x Ni _{1 + x} O _2. Can be synthesized. This Li _1-x Ni _{1 + x}
O ₂ adopts the position of the Wyckoff to obtain (Li
_1-x Ni _x ) _3a (Ni) _3b (O ₂ ) _6c That is, it has a crystal structure in which a part of the Li site is substituted by Ni.

It is expected that the Ni-rich type will cause an overcharge reaction at a low battery voltage, contrary to the Ni-deficient type. However, (Li _1-x Ni _x ) _3a (Ni)
_3b (O ₂ ) _6c is unstable and slightly excess Ni (Li ₁₋ δNiδ) _3a (Ni) _3b (O ₂ ) _6c (δ <
Since x) and NiO are easily separated, the overcharge reaction is actually started at a battery voltage that is hardly different from the normal composition, and the behavior is approximated. That is, in the lithium secondary battery of the present invention, the Ni-excess type lithium-nickel composite oxide is positioned in the same group as that of the normal composition.

The above is the operation of the lithium secondary battery in which the lithium nickel composite oxide having the ordered layered rock salt structure having the basic composition of LiNiO ₂ is used as the positive electrode active material. When M is replaced with Co or Mn, which is another element, the battery voltage causing an overcharge reaction is different. However, since the same behavior at the time of overcharge is shown, the description thereof is omitted here, and The description will be used to explain the operation of the lithium secondary battery of the present invention at the time of overcharging.

[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 Transition Metal Composite Oxide In the lithium secondary battery of the present invention, the basic composition of the positive electrode active material is LiMO ₂ (M is selected from Ni, Co, and Mn). (At least one of these) is used. A lithium-nickel composite oxide, a lithium-cobalt composite oxide having a basic composition of LiNiO ₂ , LiCoO ₂ , and LiMnO ₂ respectively;
In addition to the lithium-manganese composite oxide, the basic composition in which M is composed of two or more elements is Li (Ni, Co) O ₂ , Li
(Ni, Mn) O ₂ , Li (Co, Mn) O ₂ , Li (N
Each of the lithium transition metal composite oxides to be (i, Co, Mn) O ₂ may be used.

For the purpose of improving other battery characteristics such as thermal stability, a part of the site of the constituent element M in the crystal structure was replaced with a trivalent other metal (such as Al) at a small ratio.
For example, the composition formula LiM _1-n Al _n O ₂ (0 <n ≦ 0.1)
It does not exclude the lithium transition metal represented by. In this case, (M, Al) is put together and M
Treated as expressing.

Further, the above can be more specifically described, for example, by a composition formula LiNi _a Co _b Al _c O ₂ (a + b +
c = 1, 0 <c ≦ 0.1), the basic composition referred to in the present specification is LiM
It means that it is included in the lithium transition metal composite oxide which is O ₂ . In this case, similarly, (Ni, Co, Al)
Are treated collectively and expressed as M.

Furthermore, an ordered layered rock-salt structure lithium transition metal composite oxide having a basic composition of LiMO ₂ is not limited to a stoichiometric composition, but is deficient or excessive in an unavoidable constituent element generated in a manufacturing process. Alternatively, it also means that a mixture of other impurities is allowed.
The above description is similarly interpreted for the first positive electrode active material and the second positive electrode active material described below.

Among the above lithium transition metal composite oxides, a lithium nickel composite oxide mainly containing Ni as a constituent element M (referred to as “LiNi
Expressed as the lithium nickel composite oxide "to O ₂₎
Is excellent in that it has a large capacity, balances price with durability stability such as cycle characteristics, and can constitute a practically large lithium secondary battery. In addition, lithium secondary batteries using lithium nickel composite oxide,
Compared with the lithium secondary battery using the other composite oxide, the battery voltage that causes the overcharge reaction is low, and the overcharge reaction can be started relatively early in the overcharge,
A more secure lithium secondary battery can be configured. In consideration of this point, in the lithium secondary battery of the present invention, it is preferable that the lithium nickel composite oxide having a layered rock salt structure with a regular arrangement having a basic composition of LiNiO ₂ be used as the positive electrode active material.

(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 regular composition or a composition formula Li containing an excessive amount of the constituent element M. It is a lithium transition metal composite oxide represented by _1-x M _{1 + x} O ₂ . A composition represented by a normal composition LiMO _{2 in} which the site of the constituent element M and the Li site in the crystal structure are not substituted with Li or M, and a composition formula Li _1-x M _{1 + x} O in which the Li site is substituted with M _Two
Including those represented by

Compositional formula Li _1-x where Li site is substituted by M
When the _one represented by M _{1 + x} O ₂ is employed, the substitution ratio of M, that is, the value of x in the composition formula, is set to 0 <x ≦ 0.1.
This is because, when x> 0.1, in the normal battery voltage range, the amount of Li absorbed / desorbed during charging / discharging is excessively reduced, and the capacity of the lithium secondary battery is significantly reduced. This is because In addition, Li _1-x M _{1 + x} O ₂ is unstable and causes an overcharge reaction at a battery voltage that is almost the same as that of the normal composition, so that it is less significant to increase x.

In the above composition range, there are various lithium transition metal composite oxides having slightly different compositions. In the lithium secondary battery of the present invention, it does not prevent mixing of a plurality of types of lithium transition metal 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 Li _{1 + y1} M deficient in the constituent element M. _{'y2 M 1-y O 2} (M' is Mg, Cu, at least one selected from Zn; y1 + y2 = y) is a lithium transition metal composite oxide represented by. A composition represented by the composition formula Li _{1 + y} M _1-y O _{2 in} which a part of the site of the constituent element M in the composition having the normal composition is replaced with Li.
_One represented by a composition formula LiM ′ _y M _1-y O ₂ substituted with at least one metal element M ′ selected from Mg, Cu and Zn having a valence smaller than the valence, and substituted with both. Includes those represented by the composition formula Li _{1 + y1} M ′ _y2 M _1-y O ₂ .

Of these, those substituted with the metal element M 'are somewhat complicated in production. Therefore, in consideration of simplicity in manufacturing, a material represented by a composition formula Li _{1 + y} M _1-y O _{2 in} which a part of the site of the constituent element M is replaced with Li is used as the second positive electrode active material. It is desirable to adopt it.

The replacement ratio of the M site of the constituent element with Li and M ′, that is, the value of y in the composition formula, is set to 0 <y ≦ 0.1. This is similar to the case of the first positive electrode active material. When y> 0.1, the amount of Li absorbed / desorbed along with charging / discharging is excessively reduced in the normal battery voltage range. This is because the capacity of the secondary battery significantly decreases. According to this, considering substitution by both Li and M ′, 0 ≦ y1 ≦
0.1, 0 ≦ y2 ≦ 0.1.

Further, the battery voltage at which the overcharge reaction is started depends on the value of y. In relation to the first active material, it is preferable that the difference between the overcharge reaction start voltages of the two is 0.1 V or more. Considering this point, 0.005 ≦ y
It is desirable that ≦ 0.1.

As in the case of the first active material, there are various kinds of lithium transition metal composite oxides having slightly different compositions within the above composition range. In the lithium secondary battery of the present invention, it does not prevent the mixing of plural kinds of lithium transition metal composite oxides having slightly different compositions to form the second positive electrode active material.

(4) Method for Producing Positive Electrode Active Material Various lithium transition metal composite oxides to be used as the positive electrode active material in the lithium secondary battery of the present invention are not particularly limited. 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 these lithium transition metal composite oxides.

For example, when a lithium nickel composite oxide represented by Li _1-x Ni _{1 + x} O ₂ or a composition formula Li _{1 + y} Ni _1-y O ₂ is produced by a solid-state reaction method ,
A lithium compound serving as a Li source and a nickel compound serving as a Ni source are mixed in accordance with the composition ratio of the constituent elements of the lithium-nickel composite oxide to be synthesized, and the mixture is heated at a temperature of 700 to 900 ° C. in an oxygen atmosphere or It can be synthesized by firing in air for 8 to 24 hours. At this time, lithium hydroxide, lithium carbonate, or the like can be used as the lithium compound serving as the Li source, and nickel hydroxide, nickel nitrate, or the like can be used as the nickel compound serving as the Ni source.

(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.

When a lithium nickel composite oxide having a basic composition of LiNiO ₂ is used as a positive electrode active material, for example, a first positive electrode active material represented by a composition formula Li _1-x Ni _{1 + x} O ₂ and a composition formula Li _{1 + y1 M 'y2 Ni 1} -y O 2 with their constituent ratio when using the second positive active material represented the total of both the 1
In the case where the content of the first positive electrode active material is
It is desirable that the content be more than wt% and less than 60 wt%. This preferred range has been confirmed by experiment, and if any of the positive electrode active materials is less than this range,
This is because the effect of dispersion of the overcharge reaction is smaller than that in the range, and the overcharge reaction is dominated by the larger overcharge reaction, and the possibility of a rapid rise in temperature due to a chain reaction increases. In the case of the combination of the first positive electrode active material and the second positive electrode active material, according to verification by experiment, when the composition ratio of the first positive electrode active material is 20 wt% or more and 50 wt% or less, the battery case A lithium secondary battery in which overcharge safety is ensured because the degree of deformation is small or the like. Furthermore, in order to obtain a lithium secondary battery having an extremely low possibility of falling into a temperature rise state due to a rapid chain reaction and being more excellent in safety, the composition ratio in which the first positive electrode active material is 20 wt% or more and 30 wt% or less is required. Has been confirmed to be more desirable.

The basic composition is LiNiO_TwoRichi to be
A nickel-nickel composite oxide as a positive electrode active material;
Is partially substituted with Co and Al, for example, a composition formula
Li _1-x(Ni, Co, Al)_{1 + x}O_TwoThe first positive represented by
Extremely active material and composition formula Li_{1 + y1}M ' _y2(Ni, Co, Al)
_1-yO_TwoWhen the second positive electrode active material represented by
As in the case of the above, by experiment, their composition ratio was
Is 100 wt%, the first positive electrode active
Desirable that the substance is more than 10 wt% and less than 60 wt%
It has been confirmed that it is good. In addition, for verification by experiment
According to this, the first positive electrode active material has a content of 20 wt% or more and 50 wt% or less.
If the composition ratio is below, the degree of deformation of the battery case is small.
Lithium rechargeable battery with higher overcharge safety
Is also the same as in the above case. Furthermore, the same
In addition, the first positive electrode active material may be 20 wt% or more and 40 wt% or less.
Composition ratio, each set of the first and second positive electrode active materials
Depending on the composition, the composition is 30 wt% or more and 40 wt% or less.
It has been confirmed that the ratio is 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-mentioned positive electrode active material. Good. The above-mentioned positive electrode active material is obtained by producing each of the lithium-transition metal 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 them. For example, it can be formed by binding the powder mixture with a binder. More specifically, first, a lithium nickel composite oxide serving 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 used to secure 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.

In the lithium secondary battery of the present invention, similarly to a general lithium secondary battery, in addition to the positive electrode and the negative electrode, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte, and the like are constituent elements. . 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 contains 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 can have 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 at enhancing safety during overcharge, a safety valve for releasing the internal pressure of the battery when the internal pressure 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> The embodiment of the lithium secondary battery of the present invention has been described above. However, 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.

[0057]

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. 4 shows the cross section.

The lithium secondary battery 1 includes an electrode body 10 and
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 which are respectively joined to both open ends of the outer case 21. 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 that 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 LiNiO ₂ , and the second positive electrode active material is represented by a composition formula Li _1.05 Ni _0.95 O ₂ . N
An i-deficient lithium nickel composite oxide was used. The positive electrode active materials of the respective lithium secondary batteries have different composition ratios between the first positive electrode active material and the second positive electrode active material. 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.

[0070]

[Table 1]

<Overcharge Test> The lithium secondary batteries # 1 to # 8 were subjected to cycle charge / discharge for measuring reference capacity, which also served as conditioning. This cycle charge / discharge was performed at an environmental temperature of 25 ° C. In the first cycle, the battery was charged to a battery voltage of 4.1 V at a constant current of a current density of 0.2 mA / cm ² , and was further charged at a constant voltage at the battery voltage (total charging time of 6 hours).
Battery voltage of 3.0 at a constant current of 0.2 mA / cm ²
Discharge was performed up to V. Further, the current density was 1 mA / cm from the second cycle to the fifth cycle.
^The battery was charged at a constant current of ^{2 to} a battery voltage of 4.1 V, and the battery voltage was charged at a constant voltage (total charging time: 2 hours). Then, the battery voltage was 3.0 V at a constant current of 1 mA / cm ^2. The cycle of discharging until was repeated. Then, the discharge capacity in the fifth cycle was used as the reference capacity of each lithium secondary battery. As a result of this cycle charging and discharging, the reference capacity of each battery is:
Approximately equivalent values were shown. In addition, in any of the lithium secondary batteries, there was almost no difference in discharge capacity between the second cycle and the fifth cycle. Therefore, the normal operating voltage range of these lithium secondary batteries was 3.0 to 4.0. It was confirmed that 1 V was appropriate.

Next, each lithium secondary battery was subjected to an overcharge test. The conditions of the overcharge test were as follows. First, assuming that the current value (current value in one hour rate discharge) when discharging the reference capacity of each battery in one hour is 1 C, the battery voltage is 4.1 V at 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 this 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.

[0074]

[Table 2]

As is clear from Table 2, the safety valves are open in 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 battery voltage of the lithium secondary battery of # 8 using only the first positive electrode active material and the lithium secondary batteries of # 3 and # 4 in which even the oxidation reaction of the electrolytic solution was not observed. 5 and FIG. 6 shows the relationship between the overcharge rate and the battery temperature.

As apparent from FIGS. 5 and 6, # 8
In the lithium secondary battery described above, the overcharge reaction is started slowly, resulting in a sudden rise in temperature due to a chain reaction, which causes a significant deformation of the battery case. On the other hand, in the lithium secondary batteries of # 3 and # 4, a gradual overcharge reaction starts from the initial stage of overcharge, and the self-suppressing action of the lithium secondary battery leads to a temperature rise due to a chain reaction. You can see that it has calmed down before. Therefore, it is understood that the lithium secondary battery is extremely safe and does not cause an oxidation reaction of the electrolyte.

<Test Performed by Changing Positive Electrode Active Material> Next, a similar test was performed using a lithium nickel composite oxide having a composition different from that of the above lithium nickel composite oxide. The lithium composite oxide serving as the positive electrode active material is N
This is a lithium nickel composite oxide in which the i-site is partially substituted with Co and Al. Using this positive electrode active material, two series of lithium secondary batteries were manufactured and tested.

As the first series of 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 includes a composition formula LiNi
_A lithium-nickel composite oxide having no Ni deficiency represented by _0.81 Co _0.16 Al _0.03 O ₂ is used as the second positive electrode active material, and a Ni deficiency type oxide represented by a composition formula Li _1.02 Ni _0.79 Co _0.16 Al _0.03 O ₂ is used. A lithium nickel composite oxide was used. Note that the structure of the manufactured lithium secondary battery except for the positive electrode active material is the same as that of the above-described lithium secondary battery, and thus description thereof is omitted. The positive electrode active materials of the respective lithium secondary batteries have different composition ratios between the first positive electrode active material and the second positive electrode active material. The manufactured lithium secondary batteries are numbered # 9 to # 16, and the first in each lithium secondary battery is
The composition ratio between the positive electrode active material and the second positive electrode active material is shown in Table 3 below.

[0080]

[Table 3]

The lithium secondary batteries # 9 to # 16 were
The lithium secondary batteries were subjected to an overcharge test under the same conditions as the above overcharge test to evaluate the safety of the lithium secondary batteries. In this overcharge test, the safety of the battery was evaluated by visual observation of the state of opening 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. I gave an evaluation. The evaluation results are shown in Table 4 below.

[0082]

[Table 4]

As is clear from Table 4, the safety valves of all lithium secondary batteries are open. # 9 and # 1 using the first positive electrode active material or the second positive electrode active material alone
In the lithium secondary battery of No. 6, remarkable deformation was observed in the battery case, and it was found that there was a problem in safety. On the other hand, the first positive electrode active material and the second positive electrode active material were
In the lithium secondary batteries # 11 to # 15, although the composition ratios of the positive electrode active materials are biased, the lithium secondary batteries # 10 and # 15 are slightly deformed, but the other lithium batteries are # 11 to # 1.
No deformation of the battery case was observed in the lithium secondary battery of No. 4. Furthermore, it can be seen that the lithium secondary batteries # 12 and # 13 do not cause an oxidation reaction of the electrolytic solution, and are extremely safe for overcharge.

As the lithium secondary batteries of the second series, 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 includes a composition formula LiNi
_A lithium-nickel composite oxide having no Ni deficiency represented by _0.8 Co _0.15 Al _0.05 O ₂ is used as the second positive electrode active material, and N ₂ represented by the composition formula Li _1.02 Ni _0.79 Co _0.16 Al _0.03 O ₂ is used.
An i-deficient lithium nickel composite oxide was used. Note that the structure of the manufactured lithium secondary battery except for the positive electrode active material is the same as that of the above-described lithium secondary battery, and thus description thereof is omitted. The positive electrode active materials of the respective lithium secondary batteries have different composition ratios between the first positive electrode active material and the second positive electrode active material. The fabricated lithium secondary batteries are numbered # 17 to # 24, and the first in each lithium secondary battery is
Table 5 below shows the composition ratio between the positive electrode active material and the second positive electrode active material.

[0085]

[Table 5]

The lithium secondary batteries # 17 to # 24 were subjected to an overcharge test under the same conditions as the above-described overcharge test, and the safety of the lithium secondary batteries was evaluated. In this overcharge test, the safety of the battery was evaluated by visual observation of the state of opening 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. I gave an evaluation. The evaluation results are shown in Table 6 below.

[0087]

[Table 6]

As is clear from Table 6, the safety valves of all the lithium secondary batteries are open. # 17 and # using only the first positive electrode active material or the second positive electrode active material
In the lithium secondary battery of No. 24, a remarkable deformation was observed in the battery case, and it was found that there was a problem in safety. In contrast,
# 1 using a mixture of the first positive electrode active material and the second positive electrode active material
In the lithium secondary batteries of Nos. 8 to # 23, although the composition ratios of the positive electrode active materials are biased, the lithium secondary batteries of Nos. # 18 and # 23 are slightly deformed, but other lithium batteries are # 19 to # 23.
No deformation of the battery case was observed in the lithium secondary battery No. 22. Further, it can be seen that the lithium secondary batteries # 19 to # 21 do not cause an oxidation reaction of the electrolytic solution, and are extremely highly overcharged.

<Summary of Evaluation> Summarizing the above results, in a lithium secondary battery using a lithium transition metal composite oxide having a basic composition of LiMO ₂ as a positive electrode active material, the positive electrode 1 positive electrode active material and 2nd
It can be confirmed that, by mixing the positive electrode active material and the positive electrode active material, the overcharge reaction occurs in a time-dispersed manner, and it is possible to avoid a temperature rise due to a rapid chain reaction accompanied by a significant deformation of the battery case. 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 before the temperature rise due to the chain reaction, It can be confirmed that the lithium secondary battery has extremely excellent overcharge safety.

[0090]

According to the present invention, there is provided a lithium secondary battery using a lithium transition metal composite oxide having a basic composition of LiMO ₂ as a positive electrode active material.
The first positive electrode active material represented by _1-x M _{1 + x} O ₂ and the _second positive electrode active material represented by the composition formula Li _{1 + y1} M ′ _y2 M _1-y O ₂ Is what you do. In the lithium secondary battery of the present invention having such a configuration, since the battery voltages at the start of the overcharge reaction of the first positive electrode active material and the second positive electrode active material are different, the overcharge reaction is temporally dispersed, and A lithium secondary battery excellent in overcharge safety, in which an overcharge state can be calmed before avoiding a temperature rise due to a chain reaction or before reaching a temperature rise due to a chain reaction.

[Brief description of the drawings]

FIG. 1 shows an overcharge when a lithium secondary battery using a lithium nickel composite oxide represented by a composition formula LiNiO ₂ , which is a normal composition, as a positive electrode active material is continuously charged at a constant current to be in an overcharged state. The relationship between the rate, the battery voltage, and the battery temperature is schematically shown.

FIG. 2 shows that a lithium secondary battery using a lithium-nickel composite oxide represented by a composition formula of Li _{1 + y} Ni _1-y O ₂ deficient in Ni as a positive electrode active material is continuously charged at a constant current and excessively charged. The relationship between the overcharge rate and the battery voltage when in the charged state is schematically shown together with the case of the normal composition.

FIG. 3 shows a 1: 1 mixture of a lithium-nickel composite oxide having a regular composition and the above-described Ni-deficient lithium-nickel composite oxide.
4 schematically shows a relationship between an overcharge rate and a battery voltage when a lithium secondary battery using a mixture mixed at a weight ratio of 1 as a positive electrode active material is continuously charged with a constant current to be in an overcharged state.

FIG. 4 shows a structure of a lithium secondary battery manufactured for use in an overcharge test.

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

FIG. 6 shows the relationship between the overcharge rate and the battery temperature of the lithium secondary battery of # 8 and the lithium secondary batteries of # 3 and # 4 in Examples.

[Explanation of symbols]

 1: Cylindrical lithium secondary battery 10: Electrode body 20: Battery case 21: Outer can 22: Positive side cover plate 23: Negative side cover plate 30: Positive terminal 40: Negative terminal

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazutoshi Miwa 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory, Inc. (72) Inventor Shinobu Okayama 1 Toyota-cho, Toyota-shi, Aichi Prefecture Address Toyota Motor Corporation F term (reference) 5H029 AJ12 AK03 AL06 AL07 AL08 AL12 AM02 AM03 AM04 AM05 AM07 BJ02 BJ14 DJ17 HJ01 HJ02 5H050 AA15 BA16 BA17 CA07 CA08 CA09 CB07 CB08 CB09 CB12 FA05 FA19 HA01 HA02

Claims (3)

[Claims] The basic composition is LiMO ₂ (M is Ni, C
o, Mn), wherein the positive electrode active material is an ordered layered rock salt structure lithium transition metal composite oxide having at least one selected from the group consisting of o and Mn. Composition formula Li _1-x contained in
A first positive electrode active material represented by M _{1 + x} O ₂ (0 ≦ x ≦ 0.1); and a composition formula Li _{1 + y1} M ′ _y2 M1 _-y O ₂ (M ′ At least one selected from Mg, Cu and Zn; y1 +
0 <y1 <0.1; 0 <y2 <0.1; 0 <y
≦ 0.1), and a second positive electrode active material represented by the formula: 2. The cathode active material has a basic composition of LiNiO.
_2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is a layered rock-salt structure lithium nickel composite oxide designated as 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%.