JP2002025539A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery in which deterioration of crystal structure accompanied with charging/discharging is suppressed and the cycle property is improved by aiming at stabilization of the crystal structure of a positive electrode active substance. SOLUTION: This is a lithium secondary battery which is equipped with an electrode body 1 comprising that respective positive and negative electrode plates 2, 3 are wound or laminated via a separator 4 wherein an electrode active substance is formed on the surface of a current collecting substrate, and in which a nonaqueous electrolytic solution is used. A thermal shrinkage rate of the positive electrode active substance is made 6% or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery which realizes stabilization of the crystal structure of the positive electrode active material and, consequently, improvement of the cycle characteristics by reducing the heat shrinkage of the positive electrode active material to a predetermined value or less. The present invention relates to a lithium secondary battery capable of achieving a long life.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries have been widely used as power batteries for portable electronic devices such as mobile phones, VTRs, and notebook computers. In addition, since the lithium secondary battery has a single cell voltage of about 4 V, which is higher than that of a conventional secondary battery such as a lead storage battery, and has a higher energy density, not only the portable electronic device but also recent environmental problems. In the background, electric vehicles (EVs) are being actively promoted as low-emission vehicles
Alternatively, attention is being paid to a power supply for driving a motor of a hybrid electric vehicle (HEV).

In general, a lithium secondary battery uses a lithium transition metal composite compound as a positive electrode active material, a carbonaceous material as a negative electrode active material, and an organic electrolytic solution obtained by dissolving a Li ion electrolyte in an organic solvent as an electrolytic solution. As the electrode body for performing a battery reaction, there are a coin cell type, a wound type, and a stacked type.

Among these, in a lithium secondary battery having a relatively large capacity suitably used for an EV / HEV, etc., as shown in FIG. 1, a current collecting tab (which functions as a lead wire) is used as an electrode body. Hereafter, it is called "tab.")
The positive and negative electrode plates 2.3 (positive electrode plate 2, negative electrode plate 3) to which the 5.6 (positive electrode tab 5, negative electrode tab 6) is attached, with the separator 4 interposed therebetween so as not to contact each other. A wound electrode body 1 (hereinafter, referred to as “wound”) wound around the outer periphery of the core 13.
It is called a "wound body". ) Are preferably used.

The electrode plates 2 and 3 are formed by forming an electrode active material (refers to both a positive electrode active material and a negative electrode active material) layers on both surfaces of a current collecting substrate such as a metal foil. During the operation of winding the electrode plates 2 and 3 and the separator 4 around the winding core 13, a predetermined portion of the metal foil at the end of the electrode plates 2 and 3 is exposed by means of ultrasonic welding or the like. Can be mounted at intervals.

In addition, a laminated electrode body (hereinafter, “laminated body”)
That. 2), as shown in the perspective view of FIG. 2, has a structure in which a positive electrode plate 8 and a negative electrode plate 9 having a certain area and a predetermined shape are alternately laminated with a separator 10 interposed therebetween.
At least one tab 11, 12 is provided on one electrode plate 8, 9.
(Positive electrode tab 11, negative electrode tab 12) are attached. The materials used and the method of forming the electrode plates 8 and 9 are the same as those of the electrode plates 2 and 3 in the wound body 1.

[0007]

Here, in a lithium secondary battery for EV / HEV using the wound body 1 and the laminated body 7, it is necessary to repeatedly charge and discharge for a long period of time. Discharge cycle characteristics (Refer to battery capacity change characteristics due to repeated charge / discharge.
That. An important issue is how to minimize the degradation of the above-mentioned) and extend the battery life.

[0008] The cycle characteristics of a lithium secondary battery are largely governed by the configuration of the electrode body such as the wound body 1 and the laminated body 7, and in particular, the influence of a positive electrode active material having a smaller electron conductivity than other members. Receive greatly. Therefore, how to form the crystal structure of the positive electrode active material so as to be stable is a key to realizing a highly sustainable battery.

[0009] In addition, for EV / HEV motor drive,
Since a voltage of 100 to 300 V is required,
It is also necessary to connect a plurality of cells in series and use them as a module. In this case, since the same amount of current flows through each cell, if the crystal structure of the positive electrode active material varies greatly between cells, the cell having a large internal resistance generates heat and functions. This may cause a situation in which the entire module stops functioning.
Therefore, suppressing variation in the crystal structure of each cell,
In other words, suppressing the variation in the resistance of the electrode body is also a very important issue. It is needless to say that reduction of the internal resistance of the battery is not limited to EV use and the like, but is also an important issue from the viewpoint of maintaining good charge / discharge efficiency and cycle characteristics.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a cycle in which the thermal contraction rate of a positive electrode active material is kept at a predetermined value so that the crystal structure is less deteriorated. An object of the present invention is to provide a lithium secondary battery having excellent characteristics.

[0011]

That is, according to the present invention, there is provided an electrode body obtained by winding or laminating positive and negative electrode plates each having an electrode active material formed on the surface of a current collecting substrate with a separator interposed therebetween. And a lithium secondary battery using a non-aqueous electrolyte, wherein the positive electrode active material has a heat shrinkage of 6% or less. At this time, the heat shrinkage of the positive electrode active material is more preferably 3% or less.

The positive electrode active material suitably used in such a lithium secondary battery of the present invention has a substantially octahedral shape mainly composed of flat crystal faces as primary particles. There is no particular limitation on the electrode active material used. However, when lithium manganate having a cubic spinel structure is used as the positive electrode active material, deterioration of the crystal structure can be suppressed to a small level.

As the lithium manganate, Li
The / Mn ratio is preferably more than 0.5, and more preferably at least a part of manganese is replaced by titanium.

In the synthesis of the lithium manganate, a mixture of a salt and / or an oxide of each element adjusted to a predetermined ratio is mixed in an oxidizing atmosphere at a temperature of 650 to 1000 ° C. and a temperature of 5 to 1000 ° C.
The firing is performed for 50 hours.
By performing it twice or more times, the crystal characteristics can be improved. In this case, it is preferable that the firing temperature be higher than the previous firing temperature every time the firing is repeated, and that the pulverization treatment is performed after each firing, so that the composition can be made uniform, which is preferable.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. However, it goes without saying that the present invention is not limited to the following embodiments. The lithium secondary battery of the present invention includes an electrode body formed by winding or laminating positive and negative electrode plates each having an electrode active material layer formed on the surface of a current collecting substrate with a separator interposed therebetween, and using a non-aqueous electrolyte. It was what was.

That is, as shown in FIG. 1, the lithium secondary battery of the present invention has an electrode plate (positive electrode plate 2, negative electrode plate 3) to which a plurality of current collecting tabs (tabs) 5 and 6 are attached.・ 3
Electrode body (winding body) 1 obtained by winding the above around the outer periphery of a winding core 13 via a separator 4 or the electrode plate 8 to which the tabs 11 and 12 are attached as shown in FIG. A laminated electrode body (laminated body) 7 in which 9 (positive electrode plate 8 and negative electrode plate 9) are alternately laminated with a separator 10 interposed therebetween.

The wound body 1 and the laminate 7 have different appearances,
Since there is no difference in the materials used, the following wound body 1
The present invention will be described with reference to an example. First, the positive electrode plate 2 is manufactured by applying a positive electrode active material on both surfaces of a current collecting substrate to form a positive electrode active material layer. As the current collecting substrate, a metal foil having good corrosion resistance to a positive electrode electrochemical reaction such as an aluminum foil or a titanium foil is used, but a punching metal or a mesh (net) can be used instead of the foil.

As the positive electrode active material, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO)
A lithium transition metal composite oxide such as O ₂ ) and lithium manganate (LiMn ₂ O ₄ ) is used. In the present invention, in particular, lithium manganate having a cubic spinel structure (hereinafter referred to as “lithium manganate spinel”) is used. This is preferable because the resistance of the positive electrode active material layer can be reduced as compared with the case where another positive electrode active material is used.

The application of these various positive electrode active materials to the current collecting substrate (metal foil) is performed by adding a solvent or a binder to the positive electrode active material powder to prepare a slurry or paste (hereinafter, referred to as a slurry or paste).
It is called "slurry etc." ) Is applied to the current collecting substrate and dried using a roll coater method or the like. In addition,
In forming the positive electrode active material layer, fine carbon powder such as acetylene black or carbon black is added to the positive electrode active material powder as a conductive additive.

The negative electrode plate 3 can be manufactured in the same manner as the positive electrode plate 2. As the current collecting substrate of the negative electrode plate 3, a metal foil having good corrosion resistance to a negative electrode electrochemical reaction such as a copper foil or a nickel foil is suitably used. As the negative electrode active material, an amorphous carbonaceous material such as soft carbon or hard carbon, or a highly graphitized carbonaceous powder such as artificial graphite or natural graphite is used. Among them, in the present invention, highly graphitized carbon fibers are preferably used.

As the separator 4, a Li ion-permeable polyethylene film having micropores (PE
Film) having a three-layer structure sandwiched between porous Li ion-permeable polypropylene films (PP films) is preferably used. When the temperature of the wound body rises, the PE film softens at about 130 ° C. and the micropores are crushed, which also serves as a safety mechanism for suppressing the movement of Li ions, that is, the battery reaction. And, even when the PE film is softened by sandwiching the PE film with a PP film having a higher softening temperature,
The PP film retains its shape and prevents contact / short circuit between the positive electrode plate 2 and the negative electrode plate 3, thereby making it possible to reliably suppress battery reaction and ensure safety.

When the electrode plates 2 and 3 and the separator 4 are wound around the winding core 13, a portion of the electrode plates 2 and 3 where the current collector substrate on which the electrode active material is not applied is exposed, Tabs 5 and 6 are respectively attached. That is, it is preferable that the electrode plates 2 and 3 have a stripe structure in which the active material layer is not formed on at least one end in the width direction of the current collecting base material. The core 13 can be made of various materials such as metal, resin and ceramic. When a conductive material is used, it is necessary to ensure insulation from the electrode plates 2 and 3.

As the tabs 5 and 6, foil-like ones made of the same material as the current collecting substrate of each of the electrode plates 2 and 3 are preferably used. The tabs 5 and 6 can be attached to the electrode plates 2 and 3 using ultrasonic welding, spot welding, or the like. At this time, as shown in FIG. 1, the tabs 5 and 6 are arranged such that the tab of one electrode is arranged on one end surface of the wound body 1.
It is preferable to attach each of them to prevent contact between the tabs 5 and 6, which is preferable.

In assembling the battery, first, while ensuring conduction between the positive and negative terminals for taking out current to the outside and the tabs 5 and 6 respectively, the manufactured wound body 1 is inserted into the battery case to be stable. Hold in a proper position. After that, the battery case can be sealed by impregnating with a non-aqueous electrolyte and then sealing the battery case. In the present invention, the shape and structure of the battery case, or the tabs 5 and 6 in the wound body 1 are used.
Needless to say, there is no limitation on the form of connection between the terminal and the positive and negative terminals.

As the non-aqueous electrolyte, lithium hexafluoride
Lithium phosphate (LiPF ₆) Or lithium borofluoride (L
iBF_Four) And other lithium complex fluorine compounds or persalts
Lithium citrate (LiClO_Four) Such as lithium halide
Of one or more types selected from oxides
The quality is ethylene carbonate (EC), diethyl carbonate
Nate (DEC), dimethyl carbonate (DMC),
Carbonic esters such as propylene carbonate (PC)
Solvent, γ-butyrolactone, tetrahydrofuran,
Dissolved in a single solvent such as cetonitrile or a mixed solvent
Are preferably used.

The cycle characteristics of a lithium secondary battery formed using the above-described various materials are greatly influenced by a positive electrode active material whose electron conductivity is small as compared with other materials. In the above, a positive electrode plate having a heat shrinkage of 6% or less in the thickness direction of the positive electrode active material layer in a state not impregnated with the nonaqueous electrolyte is used. By confirming the heat shrinkage before manufacturing the rolled body, it becomes possible to obtain a battery which has a small deterioration of the crystal structure due to charge and discharge and has excellent output characteristics and durability against charge and discharge cycles.
Further, the heat shrinkage is more preferably 3% or less.

As will be described in Examples described later, when such conditions are satisfied, deterioration of the crystal structure of the positive electrode active material is suppressed, and battery characteristics are improved. More specifically, the coefficient of thermal expansion according to the present invention is a value determined and determined by a “high-precision thermal dilatometer I.2F-TMA / S model (standard type) manufactured by Rigaku Denki Co., Ltd.”.

Therefore, when the coefficient of thermal expansion is obtained by another analysis method, the value may be different from the specified value in the present invention. However, the gist of the present invention is affected by such a difference in the analysis method. Needless to say, they will not receive it.

As the positive electrode active material having a heat shrinkage ratio of 6% or less, a material having a substantially octahedral shape mainly composed of flat crystal faces as primary particles is described later. The crystal structure is stable and suitable.

Specifically, as the positive electrode active material, lithium manganate having a cubic spinel structure can be suitably used. Lithium manganate having a cubic spinel structure has a stoichiometric composition represented by LiMn ₂ O ₄ , but is not actually limited to such a stoichiometric composition, and a part of Mn that is a transition element _{1 LiM X Mn 2-X O} 4 was substituted with or more elements M (x represents a substituent.) is also suitably used. When such an element substitution is performed, the Li / Mn ratio becomes (1 + x) / (2-x) when the Mn is replaced with Li, and when the Li is excessive, the substitution element M other than Li When the substitution is made, the ratio is 1 / (2-x), so that in any case, the Li / Mn ratio is always greater than 0.1.
It becomes 5.

Among LiMn ₂ O ₄ , particularly, Li / M
When the n ratio is larger than 0.5, the crystal structure is further stabilized as compared with the case where the stoichiometric composition is used, and a battery having excellent cycle characteristics can be obtained.
preferable.

Here, as the substitution element M, Li, F
e, Mn, Ni, Mg, Zn, B, Al, Co, Cr,
Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo,
W. In addition, in the substitution element M, Li is +1 valence theoretically, and Fe, Mn, Ni, Mg, Zn are +
Divalent, B, Al, Co, Cr are trivalent, Si, Ti, S
n is +4, P, V, Sb, Nb and Ta are +5, M
o and W are +6 valent ions and are elements which form a solid solution in LiMn ₂ O _4. Co and Sn are +2 valent, and Fe, Sb and Ti are +3 valent.
For +3 and +4, for Cr +4
Valence, +6 valence. Therefore, the various substitution elements M may exist in a state having a mixed valence, and the amount of oxygen does not necessarily need to be 4 as represented by the theoretical chemical composition, and the crystal structure May be deficient or excessive in the range for maintaining the above.

The positive electrode active material having such a composition and a crystal structure further has a particle structure (particle form) as shown in a photograph (hereinafter, referred to as “SEM photograph”) obtained by SEM observation in FIG. Primary particles are mainly composed of flat crystal faces, and most of the primary particles have a substantially octahedral shape. That is, on the surface of the primary particles, smooth crystal planes intersect each other to form a clear ridge line, and the shape has a shape close to an octahedron which is an automorphic cubic spinel structure.

Here, the primary particles observed in FIG. 3 are those in which the vertices formed by intersecting the four faces of the octahedron are not completely formed, but are formed in the form of faces or edges. Or, another crystal plane appears at the ridge formed by the intersection of two octahedrons, or a certain primary particle shares the crystal plane, or a part of the surface of the primary particle , Another primary grain crystal is also observed.
However, although the shape of such primary particles is not a perfect octahedron, it can be generally regarded as an octahedral shape. In other words, it goes without saying that the “substantially octahedral shape” in the present invention includes such various shapes, and some of these shapes are missing or primary particles complicately form crystal faces. Those that share and form a polyhedron are also included.

Note that the positive electrode active material of the lithium secondary battery of the present invention is preferably composed mainly of primary particles having the above-described form, and all the primary particles have a substantially octahedral shape. It doesn't have to be. This is understood from the fact that, in general, the growth of the crystal plane may not occur uniformly due to the influence of the raw material particle size, the impurities in the raw material, the temperature distribution in the furnace during synthesis, and the like. Should be.

Next, LiM having the above-mentioned heat shrinkage rate
A method for synthesizing n ₂ O ₄ spinel will be described. As the synthesis raw material, each element (substitution element M when element substitution is performed)
And / or oxides thereof. The salt of each element is not particularly limited, but it goes without saying that it is preferable to use a high-purity and inexpensive raw material. Further, it is preferable to use a carbonate, a hydroxide, or an organic acid salt which does not generate harmful decomposition gas at the time of raising the temperature or firing. However, nitrates, hydrochlorides and sulfates can also be used. In addition, about a Li raw material, usually,
Oxide Li ₂ O is rarely used because it is chemically unstable, and hydroxide and carbonate are preferably used.

A mixture of such raw materials at a predetermined ratio is first mixed in an oxidizing atmosphere at 650 to 1000 ° C. for 5 minutes.
Bake for ~ 50 hours. Here, the oxidizing atmosphere is
Generally, it refers to an atmosphere having an oxygen partial pressure at which an in-furnace sample causes an oxidation reaction, and specifically, an air atmosphere, an oxygen atmosphere, or the like.

After the first baking, the uniformity of the composition is not always good, the crystallite is small, and the lattice distortion tends to be large. However, when Li / Mn ratio> 0.5 is satisfied, that is, when the stoichiometric composition is replaced with Mn element, Li or Ti particularly
In an excess Li composition obtained by partially substituting n, 1
It has been experimentally confirmed that the crystallite size and / or lattice strain can be more easily satisfied with the predetermined conditions by sintering twice. Although the reason for this is not clear, the addition of the substitution element M stabilizes the crystal lattice, and changes the phase atmosphere during synthesis to change the atmosphere suitable for crystal growth, for example,
It is presumed that the liquid-phase atmosphere and the gas-phase atmosphere are likely to appear.

As described above, in some compositions, the crystallite size and / or the lattice strain can be made to satisfy predetermined conditions by one firing, but in order to reduce the composition dependency of the synthesis conditions. In this case, it is preferable to perform the firing in a plurality of times. In this case, from the viewpoint of uniformity of the composition, it is more preferable to perform a pulverization treatment after the first baking, and then perform the second and subsequent baking. The number of firings largely depends on the firing temperature and the firing time. When the firing temperature is low and / or when the firing time is short, many firings are required. Also, depending on the type of the substitution element M, it may be preferable to increase the number of firings from the viewpoint of making the composition uniform. In this case, it is considered that it is difficult to form a phase atmosphere suitable for crystal growth by adding the substitution element M.

However, since increasing the number of firings means that the production process becomes longer, it is preferable to keep the number of firings to the minimum necessary. Since the sample obtained by performing such multiple firings has a sharper peak shape on the XRD chart than the sample obtained by performing single firing, the crystallinity is improved. It can be confirmed that it is planned.

The pulverization treatment is performed after each firing, but the method is not limited, and for example, can be performed using a ball mill, a vibration mill, an airflow pulverizer, or the like. The grinding process also
Although it also contributes to the uniformity of the particle size, it is preferable to perform the pulverization treatment so that the average particle size of the pulverized sample is 10 μm or less in order to sufficiently obtain the effect of the uniformity of the composition. In addition,
The average particle size in this case, the powder is ultrasonically dispersed in distilled water,
This is a value measured using a laser diffraction method.

When the firing temperature is as low as less than 600 ° C., a peak indicating the residual raw material in the XRD chart of the fired product, for example, lithium carbonate (Li ₂ C
When O ₃ ) is used, a peak of Li ₂ CO ₃ is observed, and a single-phase product cannot be obtained. On the other hand, if the firing temperature is 100
When the temperature is higher than 0 ° C., a high-temperature phase is generated in addition to the target crystalline compound, and a single-phase product cannot be obtained.

As described above, by using the positive electrode active material satisfying the conditions of the present invention and having a heat shrinkage of 6% or less, the crystal structure of the positive electrode active material is stabilized, and the crystal structure accompanying charge / discharge is achieved. Is improved, and the deterioration of the charge / discharge efficiency of the battery is suppressed. In this way, an excellent effect of suppressing a reduction in battery capacity over time due to repeated charging and discharging is obtained.

[0044]

EXAMPLES Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

(Synthesis of Positive Electrode Active Material LiMn ₂ O _4, etc.) As starting materials, commercially available Li ₂ CO ₃ , MnO ₂ , TiO ₂ , M
gO powder was weighed and mixed so as to have the compositions of Examples 1 to 3 and Comparative Example 1 shown in Table 1 and fired in the air under the first firing conditions also shown in Table 1. Example 2
For, the powder obtained by the first baking was used as a sample. On the other hand, in Example 1, Example 3, and Comparative Example 1, after the first firing, firing was performed under the second firing conditions shown in Table 1 to obtain samples. In addition, about Mg and Ti, the mixing ratio was set to Mg: Ti = 1: 1.

[0046]

[Table 1]

(Measurement of Thermal Expansion of Positive Electrode Material) The coefficients of thermal expansion of the obtained various samples were measured with a high-precision thermal dilatometer (IS.2F-TMA).
/ S model (standard type), manufactured by Rigaku Denki Co., Ltd.)
Measurement temperature range R. T. ~ 1000 ° C, heating rate 5 ° C / m
in, a thermocouple for temperature control PtRh13% R type, measured by a differential expansion measurement method in an air atmosphere. In determining the coefficient of thermal expansion, transparent quartz (for thermal analysis) was used as an external standard sample. Measurement sample is transparent quartz 5mmφ
Each sample was placed in a sample container of 50 mm, transparent quartz as a standard sample was set in the apparatus, and measurement was started. As a result, a thermal expansion curve was obtained.

(Method of Calculating Thermal Shrinkage of Positive Electrode Material) The coefficient of thermal expansion was determined as shown in the following (1) to (4) by defining a reference as shown in FIG. (1) Let T1 be the temperature of the maximum expansion coefficient of the thermal expansion coefficient curve, and let L be the expansion coefficient at that time.
It was set to 1. (2) After the sample shows the maximum coefficient of thermal expansion, contraction starts, and a temperature 100 ° C. higher than T1 is defined as T2, and T2
The expansion coefficient at was defined as L2. (3) The difference between the expansion coefficients L1 and L2 was determined, and the difference was defined as ΔL (= L1−L2).
(4) The heat shrinkage was calculated from ΔL / L1 (%).

(Preparation of Battery) Various prepared samples, acetylene black powder as a conductive material, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 50: 2: 3 to prepare a positive electrode material. . 0.02 g of the positive electrode material
It was press-formed into a disc having a diameter of 20 mmφ under a pressure of 00 kg / cm ² to obtain a positive electrode. A negative electrode composed of an electrolyte prepared by dissolving LiPF ₆ as an electrolyte at a concentration of 1 mol / L in an organic solvent in which ethylene carbonate and diethyl carbonate are mixed at an equal volume ratio to a concentration of 1 mol / L; A coin cell was prepared using a separator separating the cells.

(Method for Evaluating Cycle Characteristics) Next, a cycle test in a coin cell type battery will be described. In the present invention, according to the capacity of the positive electrode active material, the produced coin cell is charged at a constant current-constant voltage until a voltage of 4.1 V is reached at a current corresponding to 1 C, and then discharged at a constant current also at a current equivalent to 1 C, A pattern for discharging until the voltage becomes 2.5 V is set, and this charge / discharge cycle is defined as one cycle.
The test was performed by repeating this. The cycle characteristics (%) were evaluated by calculating the capacity ratio by dividing the discharge capacity at the 100th cycle by the discharge capacity at the first cycle (first time). The results are shown in Table 2.

[0051]

[Table 2]

(Test Results) The thermal expansion curves of Example 1 and Comparative Example 1 are shown in FIGS. 4 (b) and 4 (c). As can be seen from Table 2, the batteries of Examples 1 to 3 according to the present invention achieved a capacity retention of 80% or more and a heat shrinkage of more than 6% in 100 cycle tests. The charge / discharge cycle characteristics better than No. 1 were exhibited. This is because the positive electrode active material having a small heat shrinkage in the present invention has a strong bonding force between the constituent elements of the crystal, so that the crystal structure is stable,
It is considered that the deterioration of the crystal structure of the positive electrode active material due to charge / discharge due to insertion / removal of Li ions was suppressed, so that the charge / discharge cycle life was improved.

In Comparative Example 1, the Li / Mn ratio was 0.1.
In contrast, the lithium manganate having an Li / Mn ratio of more than 0.5 has a small heat shrinkage rate and a stable crystal structure, as compared with Example 5. Was obtained. Further, a comparison between Examples 1 and 2 and Example 3 showed that the compound-substituted lithium manganate exhibited better charge / discharge cycle characteristics than the Li-rich type. In addition, according to the comparison between Example 1 and Example 2, when the firing was performed twice more than once, the heat shrinkage of the positive electrode active material was reduced, and the result was that the charge / discharge cycle was excellent. This is 2
It is considered that the composition was made more uniform, the crystallinity was improved, and the bonding force between the constituent elements became stronger by performing the firing twice.

[0054]

As described above, according to the lithium secondary battery of the present invention, the use of a positive electrode active material having improved crystal structure stability and a small heat shrinkage ratio allows the use of Li ions during charging and discharging of the battery. This leads to an excellent effect of improving the insertion / removal of, thereby improving the charge / discharge cycle characteristics.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the structure of a wound internal electrode body.

FIG. 2 is a perspective view showing a structure of a laminated internal electrode body.

FIG. 3 is an SEM photograph showing one mode of a particle structure of a positive electrode active material suitably used for a lithium secondary battery of the present invention.

FIG. 4 is a schematic diagram of a thermal expansion coefficient curve (a) in the present invention;
(B) Example 1 and (c) Comparative Example 1.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Wound electrode body, 2 ... Positive electrode plate, 3 ... Negative electrode plate, 4 ... Separator, 5 ... Positive electrode tab, 6 ... Negative electrode tab, 7 ... Laminated electrode body, 8 ... Positive electrode plate, 9 ... Negative electrode plate , 10 ... separator,
11: tab for positive electrode, 12: tab for negative electrode, 13: core.

Continued on front page F-term (reference) 5H029 AJ05 AK03 AL06 AM03 AM04 AM05 AM07 BJ02 BJ04 BJ12 BJ14 CJ02 CJ08 CJ28 DJ16 DJ17 HJ00 HJ01 HJ02 HJ14 5H050 AA07 CA09 CB07 EA10 EA24 FA17 FA19 GA02 GA05 HA10 GA26

Claims (10)

[Claims] A lithium secondary battery comprising an electrode body formed by winding or laminating positive and negative electrode plates each having an electrode active material formed on the surface of a current collecting substrate with a separator interposed therebetween, and using a non-aqueous electrolyte. A lithium secondary battery, wherein the heat shrinkage of the positive electrode active material is 6% or less. 2. The lithium secondary battery according to claim 1, wherein the heat shrinkage of the positive electrode active material is 3% or less. 3. The lithium secondary battery according to claim 1, wherein the positive electrode active material has a substantially octahedral shape mainly composed of flat crystal faces as primary particles. 4. The lithium secondary battery according to claim 1, wherein lithium manganate having a cubic spinel structure is used as the positive electrode active material. 5. The method according to claim 5, wherein Li /
The lithium secondary battery according to claim 4, wherein the Mn ratio is more than 0.5. 6. The lithium secondary battery according to claim 4, wherein the lithium manganate is obtained by replacing at least a part of manganese with titanium. 7. The lithium manganate is a salt and / or oxide mixture of each element adjusted to a predetermined ratio,
The lithium secondary battery according to any one of claims 4 to 6, which is obtained by firing in an oxidizing atmosphere at a temperature of 650 to 1000 ° C for 5 to 50 hours. 8. The lithium secondary battery according to claim 4, wherein the lithium manganate is obtained by performing the firing at least twice or more. 9. The lithium according to claim 4, wherein the lithium manganate is obtained by successively increasing the firing temperature each time the firing is repeated. Rechargeable battery. 10. The lithium secondary battery according to claim 9, wherein a pulverization process is performed after each firing.