JP2002008660A - Positive electrode active material for lithium secondary battery and its manufacturing method, and lithium secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a fall of charge/discharge capacity and volume energy density by having no phase change of rhombic crystal even if charge/discharge is repeated, or even if phase change exits, by stabilizing a spinel-phase-like 2nd phase generated by phase change, with constant invertion ratio. SOLUTION: A composite expressed with LiAxByMn1-x-yO2 is included by substituting a part of Mn of a manganese acid lithium compound with an element A having a larger ionic radius than Mn and an element B having a smaller ionic radius than Mn. However, A is the element of either Sn or Pb or both elements, and B is one sort or two sorts or more of elements chosen from a group which consists of Ti, Cr, Fe, and Al. Moreover, 0<x<0.3, 0<y<0.3, and 0<(x+y)<0.3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material used for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the positive electrode active material.

[0002]

2. Description of the Related Art Generally, the concept of a lithium secondary battery is as follows.
As shown in FIG.
The electrolyte 3 is stored in the container 2 partitioned into a and a second chamber 2b, the positive electrode 4 is accommodated in the first chamber 2a in a state of being immersed in the electrolyte 3, and the negative electrode 5 is further accommodated in the second chamber 2b. It is structured to be accommodated in a state of being immersed in the electrolyte 3. In the lithium secondary battery 1, the positive electrode 4 is such that a slurry containing an active material is wetted or impregnated on an aluminum mesh plate, and then the wetted material or impregnated material is heated and dried to attach the active material to the aluminum mesh plate. The negative electrode 5 is formed in a plate shape from carbon or metal lithium represented by graphite or the like.

As the active material to be attached to the positive electrode 4, LiCoO ₂ or LiNiO ₂ has been conventionally used. In recent years, orthorhombic LiMnO ₂ has been known to function as a positive electrode active material for a lithium secondary battery. Has been reported (I.
Koetsushau, et.al., J. Electrochem. Soc., 142 (1995) 2906
-2910). However, when the positive electrode active material composed of the orthorhombic LiMnO ₂ is attached to a positive electrode, and the positive electrode is incorporated in a lithium secondary battery and charging and discharging are repeated, the composition formula is increased as the charging and discharging cycle progresses. Generation of a spinel phase having a crystal structure belonging to a cubic system represented by LiMnO ₄ , or a phase in which the spinel phase is considered to be distorted into tetragonal crystal proceeds, and the charge and discharge characteristic of the active material described above. It has been pointed out that high capacity is gradually lost (I.Koetsush
au and JR Dahn, J. Electrochem. Soc., 145 (1998) 2672-2
677).

To solve this problem, Japanese Patent Laid-Open Publication No. Hei 6-34
No. 9494 discloses a composition stabilized by adding an element to orthorhombic LiMnO ₂ , that is, a composition formula Li _x A _y M
a compound represented by nO _z (where A is H, Na, K, M
g, Ca, Sr, Ti, V, Cr, Fe, Ni, Co, and at least one element selected from the group consisting of Al, 0 <x <1.5, 0 <y <1 and 2
<Z <3. )), A method for producing a solid solution material for a non-aqueous secondary battery is disclosed. In this method for producing a solid solution material for a nonaqueous secondary battery, a positive electrode active material stabilized by adding an element is attached to a positive electrode, and charge and discharge are repeated in a lithium secondary battery incorporating the positive electrode. Since the orthorhombic LiMnO ₂ is stabilized, the formation of the spinel phase is prevented, and the cycle life of the lithium secondary battery can be improved.

In Japanese Patent No. 2547137, although not identified as orthorhombic, as a method for improving the charge-discharge cycle characteristics of a Li—Mn—O-based active material, Mn oxide containing Mn oxide contains Mn oxide. -A positive electrode active material for a lithium secondary battery comprising a Li compound, in which a part of Mn is replaced with one or both elements of Mo and W belonging to Group 6A of the periodic table, that is, a plurality of positive electrode active materials. A positive electrode active material that is effective in improving characteristics by substitution with an element is disclosed. It is considered that in this positive electrode active material, the crystal structure of the Mn-Li composite is stabilized by substitution with a plurality of elements, and thus the charge-discharge cycle characteristics are enhanced. Further, Japanese Patent Laid-Open No. 8-78007, in order to improve the discharge capacity of the positive electrode active material LiNiO ₂ based, the complex oxide having a layered structure represented by ^{_{Li a Ni b M 1 c M}} 2 d O 2 of, M ¹ is Mn, Fe, T
i, V, Cr or Cu, wherein M ² is one or two selected from the group consisting of Al, In and Sn
Lithium secondary batteries that are more than one element are disclosed. In this lithium secondary battery, Mn, Fe, Ti, V, Cr, and Cu are added when a part of Ni is replaced by an element such as Al, In, or Sn as one of means for solving the problem.
It is described that a substituted compound can be easily produced.
In the present invention, the fact that substitution with two elements having different ionic radii facilitates substitution is utilized as a result in the present invention. However, in the above publication, when substitution with two elements suppresses repulsion between oxygen atoms during charging. It is described that, since it is possible, the layered structure can be stabilized, that is, the discharge capacity can be improved.

[0006]

However, in the conventional method for producing a solid solution material for a non-aqueous secondary battery disclosed in Japanese Patent Application Laid-Open No. 6-349494, a positive electrode stabilized by adding a predetermined element is used. An active material was obtained, and this positive electrode active material could improve the initial charge / discharge capacity as compared with the unadded LiMnO ₂ , but the charge / discharge cycle characteristics were not yet improved. The reason why the charge / discharge cycle characteristics are not improved is that when the valence of Mn changes between trivalent and tetravalent in the charge / discharge reaction, the Jahn-Teller effect (the valence of the element changes, and Strain due to the expansion and contraction of the crystal grains), and the strain of the crystal grains caused by the volume change caused by the progress of the phase transformation from the orthorhombic phase to the spinel phase second phase. Is accumulated each time charge and discharge cycles are repeated, and these strains change so as to inhibit the electrical connection between the active materials and the electrical connection between the active material and the conductive material, or the spinel-like phase generated by the phase transformation. Since the two phases do not contribute to charge and discharge, it is considered that the charge and discharge capacity gradually decreases as the charge and discharge cycle progresses.

Further, in the positive electrode active material for a lithium secondary battery disclosed in the above-mentioned conventional Japanese Patent No. 2547137, since the measurement voltage range is 2 to 3.8 V, the discharge potential of the active material is 3.8 V or less. There is a problem that the volume energy density of the active material cannot be increased. Further, in the lithium secondary battery disclosed in the above-mentioned conventional Japanese Patent Application Laid-Open No. 8-78007, the positive electrode active material is Ni-based, and the crystal structure is Li _a.
Since a layered structure represented by ^{_{Ni b M 1 c M 2 d}} O 2, when repeated charge and discharge, is also considered that similarly to the above charge-discharge capacity decreases gradually.

It is an object of the present invention to provide a method in which, even when charging and discharging are repeated, there is no phase change of the orthorhombic system, or even if the phase change, the spinel-phase second phase generated by the phase change is stable at a constant conversion. It is an object of the present invention to provide a positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same, which can be made to be capable of suppressing a decrease in charge / discharge capacity and volume energy density.

[0009]

Means for Solving the Problems The present inventors have thought that if a sufficient path for Li ions moving in the active material during charging and discharging can be ensured, both the charging and discharging cycle characteristics and the discharge capacity can be improved. In consideration of this, elements to be replaced were narrowed down by the following approaches. In response to the charge / discharge cycle, the active material releases Li ions during charging and takes in Li ions during discharging. The inside of the crystal of the active material has a layered structure in which a layer composed of an octahedron in which Mn is coordinated to oxygen and a layer composed of an octahedron in which Li is coordinated to oxygen are mutually stacked. I have. In the charging reaction, Li cuts off the bond with oxygen and conducts to the surface of the active material while passing through the closest surface of oxygen atoms (corresponding to the octahedral surface). Conversely, the surface of the active material in the discharging reaction From the inside, Li conducts toward the inside of the active material so as to fill the voids of Li.

Therefore, it is considered that the cycle deterioration of the discharge capacity is caused by the inhibition of the conduction of Li into the active material. Here, one of the representative active materials, LiNiO ₂ [Hiramo.et.al, Solid.stat
e.ionics.86 / 88 (1996) 791] and one of the objects to be developed, monoclinic LiMnO ₂ [ARArmstrong & P.G.Brice, Nature3
81 (1996) 499], the difference in the structure was compared and found.
In the conduction inside the active material i, it was found that there was a difference in the spacing of oxygen atoms in the closest plane of oxygen atoms serving as barriers. The radius of the gap between the closest oxygen surfaces of LiMnO ₂ is 0.1.
264 °, and the radius of the gap between the closest oxygen surfaces of LiMnO ₂ was 0.244 °. The spacing of oxygen atoms in the oxygen closest plane that is most suitable for Li conduction is LiNiO ₂ or LiNiO _2.
Although different for MnO ₂ , it is assumed that if the barrier to the conduction phenomenon of Li is reduced, Li is smoothly conducted in the discharge reaction.

[0011] Further, if the conduction of Li occurs easily, it is considered that the interaction with the structure of the active material is reduced in the activation process over the barrier, and the stability of the structure itself is increased. Therefore, utilizing the fact that oxygen is also bonded to Mn at the same time as Li, by substituting a part of Mn with an element, the distance between the substituted atom and oxygen changes, so that oxygen in the densest oxygen plane is changed. Indirect control of the spacing of the oxygen atoms in the compound was considered. In other words, in order to obtain the interval of oxygen atoms in the oxygen closest plane most suitable for Li conduction, Mn and the average ionic radius of the atom substituted for Mn are used as parameters to increase the average ionic radius. As a result of selecting a substitution element and confirming it by an experiment, the present invention has been achieved.

The invention according to claim 1 is an improvement of a lithium manganate compound. The characteristic configuration is as follows: a part of Mn is replaced with an element A having an ionic radius larger than Mn and an element B having an ionic radius smaller than Mn, and the following formula (1) is obtained.
And a composition represented by the formula: _{LiA x B y Mn 1-xy} O 2 ...... (1) where, A is an element of one or both of Sn or Pb, B is selected from the group consisting of Ti, Cr, Fe and Al 1 Species or two or more elements, and 0 <x <0.
3, 0 <y <0.3, and 0 <(x + y) <
0.3.

In the positive electrode active material for a lithium secondary battery according to the first aspect, the volume change accompanying the charge / discharge cycle is small, that is, the phase change hardly occurs and the second phase formed by the phase transformation is charged. It shows a discharge reaction. As a result,
Even if charge / discharge is repeated, there is no phase change of the orthorhombic phase, or even if there is, the spinel phase-like second phase generated by the phase change can be stabilized at a constant conversion. A reduction in capacity can be suppressed, and a volume energy density does not decrease. Further, it is preferable that the crystal system before the first charge is indexed by an orthorhombic system. Further, in the above formula (1), A is one or both elements of Sn or Pb, B is Cr, and 0.00
It is preferable that 1 ≦ x ≦ 0.07, 0.001 ≦ y ≦ 0.1, and 0.02 ≦ (x + y) ≦ 0.17.

According to a third aspect of the present invention, when the total amount of the objects to be weighed is 100% by weight, an oxide or acetate of Mn is added.
9 to 5.3% by weight, and an additive C0. Composed of one or both of oxides, hydroxides, chlorides and acetates of Sn or Pb, or a mixture thereof.
0.01 to 1.3% by weight and one or more oxides, hydroxides or chlorides selected from the group consisting of Ti, Cr, Fe and Al, or a mixture thereof. Weighing 0.15 to 0.3% by weight of the additive D; immediately after weighing, or 750 to 850 ° C. using the oxide or acetate of Mn and one or both of the additives C and D. After calcination in air at a predetermined temperature for 5 to 25 hours to produce an intermediate, lithium hydroxide monohydrate is added to a mixture of weighed substances, or a mixture of weighed substances and intermediate, or Mn. At a predetermined temperature of 140 to 280 ° C.
A method for producing a positive electrode active material for a lithium secondary battery, comprising: a step of reacting under hydrothermal conditions for holding for up to 30 hours; and a step of washing the reaction product with water or ethanol and then drying it in a vacuum. By manufacturing the positive electrode active material by the method described in claim 3, the positive electrode active material described in any one of claims 1 to 3 can be obtained. Claim 1
By manufacturing a lithium secondary battery using any of the positive electrode active materials described in any one of (3) to (3), it is possible to suppress a decrease in the charge / discharge capacity of the lithium secondary battery.

[0015]

Embodiments of the present invention will now be described with reference to the drawings. As shown in FIG. 1, the lithium secondary battery 10 is a sheet-like laminate in this embodiment,
A positive electrode current collector plate 11, a positive electrode 12 containing a positive electrode active material, an electrolyte sheet 13, a negative electrode 14 containing a negative electrode active material, and a negative electrode current collector plate 15 are laminated in this order. The positive current collector 11 is made of an aluminum plate, and the negative current collector 15
Consists of a copper plate. As the positive electrode active material included in the positive electrode 12, a lithium manganate compound indexed to orthorhombic is used, and as the negative electrode active material included in the negative electrode 14, a graphite active material is used. Further, as the electrolyte sheet 13, a polyethylene oxide sheet containing an electrolyte is used.

On the other hand, part of Mn of the lithium manganate compound used as the positive electrode active material is replaced with an element having an ionic radius larger than Mn and an element B having an ionic radius smaller than Mn, and the following formula ( It contains the composition represented by 1). _{LiA x B y Mn 1-xy} O 2 ...... (1) where, A is an element of one or both of Sn or Pb, B is Ti, Cr. One or more elements selected from the group consisting of Fe and Al. Also, 0 <x
<0.3, preferably 0.01 <x <0.12, and 0
<Y <0.3, preferably 0.01 <y <0.15, and 0 <(x + y) <0.3, preferably 0.02 <(x
+ Y) <0.17.

The element A does not change its valence in the orthorhombic LiMnO ₂ and does not directly contribute to the lithium ion insertion / desorption reaction (charge / discharge reaction). For this reason, when the element substitution at a high concentration is performed, the effective charge / discharge capacity of the positive electrode active material decreases. Therefore, from a practical viewpoint, the necessity of suppressing the decrease in the effective charge / discharge capacity to within 30% and the solid solubility limit are reduced. As a result of consideration,
The upper limit of substitution was less than 0.3, that is, x <0.3. Since the valence of Al in the element B does not change in the orthorhombic LiMnO ₂ , the upper limit of substitution is set to 0.3 for the above-mentioned reason.
Less, that is, y <0.3. The element B other than Al changes its valence in the orthorhombic LiMnO ₂ and can contribute to the insertion / desorption reaction of lithium ions. However, there is a problem of the solubility limit to the orthorhombic crystal. Less than 0.3, ie, y <
0.3. Further, when the elements A and B are mixed and substituted, the upper limit is also less than 0.3, that is, (x + y) in order to satisfy the requirements of maintaining the effective charge capacity and maintaining the crystal structure.
<0.3.

Even within the above range of the substitution composition, depending on the combination of the elements selected from the group of the two elements A and B, some of the elements are mainly precipitated from the orthorhombic form in the form of oxides, Tests have shown that a mixture of tetragonal crystals and precipitates may form. In this case, when the obtained mixture of the orthorhombic crystal and the precipitate is examined in detail, the elements A and B are certainly dissolved in the orthorhombic crystal, but the elements A and B exceed the solid solubility limit. Has been found to precipitate mainly in the form of an oxide. The crystal system (before the first charge) of the positive electrode active material is preferably indexed in an orthorhombic system. That is, it is preferable that a diffraction peak obtained by measuring the obtained powder of the positive electrode active material by powder X-ray diffraction is identified by an orthorhombic plane index.

Further, the element A is preferably one or both of Sn and Pb, and the element B is C
r is preferred. In this case, 0.001 ≦ x ≦ 0.07
More preferably, 0.01 ≦ x ≦ 0.05, and 0.0
01 ≦ y ≦ 0.1, more preferably 0.01 ≦ y ≦ 0.0
6, and 0.02 ≦ (x + y) ≦ 0.17, more preferably 0.02 ≦ (x + y) ≦ 0.11. 0.00
The reason for limiting to 1 ≦ x ≦ 0.07 is that if less than 0.001, the effect of stabilizing the charge / discharge cycle characteristics cannot be obtained.
If it exceeds 0.07, the amount of oxides such as Sn increases, and the discharge capacity per unit weight relatively decreases. Also, the reason for limiting to 0.001 ≦ y ≦ 0.1 is that if it is less than 0.001, Sn in the LiMnO ₂ compound
Cannot be dissolved, that is, the effect of stabilizing the charge / discharge cycle characteristics cannot be obtained.
This is because the amount of oxide precipitates increases, and the discharge capacity per unit weight relatively decreases. Further 0.01
The reason for limiting to ≦ x ≦ 0.05 is that if it is less than 0.01, the volume energy density is low, and if it exceeds 0.05, the volume energy density hardly changes.

A method for producing the positive electrode active material thus configured will be described. First, assuming that the total weight of the objects to be weighed is 100% by weight, 2.9 to 5.3% by weight of Mn oxide or acetate, preferably 3.5 to 4.5% by weight, and oxide of element A, chloride C or 0.01 to 1.3% by weight, preferably 0.05 to 1.0% by weight of an additive C composed of any one of a compound and an acetate or a mixture thereof, and an oxide, hydroxide or chloride of the element B. 0.15 to 0.3% by weight, preferably 0.16 to 0.25% by weight of an additive D constituted by any of these substances or a mixture thereof. Then, immediately after weighing, or the oxide or acetate of Mn and one or both of additives C and D are 750 to 8
Calcination in air at a predetermined temperature of 50 ° C. for 5 to 25 hours produces an intermediate. Next, lithium hydroxide 1 was added to the mixture of the above weighed substances, or the mixture of the weighed substances and the intermediate, or the intermediate.
The hydrate is added so that the ratio of Li to Mn is 30 to 60 times, and at a predetermined temperature of 140 to 280 ° C, 2 to 3 times.
The reaction is carried out under hydrothermal conditions maintained for 0 hours (hydrothermal synthesis method). Further, the reaction product is washed with water or ethanol and then dried under vacuum. Thereby, the positive electrode active material is manufactured.

As Mn oxides or acetates, Mn ₂
O ₃ , MnOOH, Mn (CH ₃ COO) ₂ .4H ₂ O, etc., and the average particle size is preferably 5 to 50 μm. Examples of the additive C include SnO ₂ and PbO ₂ , and the average particle size is preferably 8 to 60 μm. Further, as additive D, Cr ₂ O ₃ , TiO ₂ , Fe
OOH, Al (OH) _3, etc., and the average particle size is preferably 10 to 30 μm. The average particle size of the positive electrode active material manufactured as described above is 0.3 to 1
It is preferably 0 μm. In the hydrothermal synthesis in which Li is charged 40 times the amount of Mn, 2.9 to 5.
The reason why the content is limited to 3% by weight is that if the content is less than 2.9% by weight, Li becomes excessive relative to Mn, crystal grains become coarse and the discharge capacity decreases, and 5.3% by weight. %, Li becomes relatively insufficient with respect to Mn.
This is because there is a problem that the n-oxide precipitates as an excess and the discharge capacity decreases. Further, the content of the oxide or the like of the additive C is set to 0.
The reason why the content is limited to 0.01 to 1.3% by weight is that if it is less than 0.01% by weight, there is no effect on the improvement of the charge / discharge cycle characteristics, and 1.3% by weight.
If the content exceeds 10% by weight, an oxide such as Sn precipitates, and the discharge capacity per unit weight relatively decreases. Further, the reason why the content of the oxides of the additive D and the like is limited to 0.15 to 0.3% by weight is that if the content is less than 0.15% by weight, there is no effect on the improvement of the charge / discharge cycle characteristics. This is because oxides such as Cr precipitate and the discharge capacity per unit weight relatively decreases.

A method for calcining one or both of the oxide or acetate of Mn and the additive C or D at a temperature of 750 to 850 ° C. for 5 to 25 hours to form an intermediate. The compounds include SnO ₂ and PbO _{2 in the} additive C group.
In the group of additive D, FeOOH, Fe ₂ O ₃ , C
r ₂ O _{3 and the} like.

It is preferable to use a reaction vessel made of a fluororesin. Lithium hydroxide monohydrate is added so that the ratio of Li to Mn is 7 to 60 times,
If it is less than 7 times, conversion to the orthorhombic LiMnO ₂ structure does not sufficiently occur, and if it exceeds 60 times, there is no difference in the effect on the quality and form of the product even if it is added. More preferably, the amount of lithium hydroxide monohydrate added is 35 to 45 times. The reason why the reaction temperature under hydrothermal conditions is limited to 140 to 280 ° C. is that the reaction temperature is 14
If the temperature does not reach 0 ° C., conversion to the LiMnO ₂ structure belonging to the orthorhombic system does not sufficiently occur. If the temperature exceeds 280 ° C., a critical state is approached in the synthesis using an autoclave. The reaction temperature under this hydrothermal condition is 200 to 2
More preferably, the temperature is 50 ° C. Furthermore, the reason why the reaction time under hydrothermal conditions was limited to 2 to 30 hours is that conversion to the LiMnO ₂ structure did not sufficiently occur if the reaction time was less than 2 hours.
This is because there is no particular difference in the manufactured positive electrode active material even when the positive electrode active material is maintained for more than 0 hours. The reaction time under this hydrothermal condition is more preferably 4 to 6 hours.

In the positive electrode active material thus manufactured,
The formation of the spinel phase accompanying the progress of the charge / discharge cycle can be effectively prevented. The following two reasons can be considered as the reasons that contributed most to the improvement of the charge / discharge cycle characteristics. The first reason is considered to be the stabilization of the phase of orthorhombic LiMnO ₂ . The bonding distance between metal ions and oxide ions of orthorhombic LiMnO ₂ is LiCoO ₂ or LiN.
Shorter than iO ₂ , it is considered that the barrier energy of the diffusion path of lithium ions is higher than other positive electrode active materials. By substituting a part of Mn with an element having an ionic radius larger than Mn, the average value of the bonding distance between the metal ion and the oxide ion becomes slightly larger, and the energy of the diffusion barrier for lithium ions is slightly reduced. This is considered to be because an active material in which lithium easily diffuses and whose charge and discharge were stable was obtained.

Here, in elemental substitution, simply by substitution with an element having a larger ionic radius than Mn, a desired solid solution cannot be formed. Need to be replaced. This is considered to be because the distortion caused by the substitution with the element having the large ionic radius can be alleviated by the substitution with the element having the small ionic radius, and the solid solubility limit is widened. The second reason is considered to be that the spinel phase-like second phase generated as a result of the phase transformation of the orthorhombic LiMnO ₂ contributes to the charge / discharge reaction.

As general knowledge, what is called LiMn ₂
It is known that it is effective to reduce the average ionic radius of the atoms occupied by Mn in order to stabilize the charge / discharge characteristics of the spinel phase having the composition formula of O ₄ . Specifically, it is a composition in which a part of Mn is substituted by A1. When the constituent phases of the active material contained in the electrode after charging and discharging the orthorhombic LiMnO ₂ of the present invention are to be identified by X-ray diffraction measurement, the constituent phases of the active material are obliquely affected by the binder and the conductive additive constituting the positive electrode. It is difficult to measure the lattice constants of the crystalline LiMnO ₂ phase and the second phase, and it is still unclear how the substitution elements are present in each active material. It is considered that Cr, Al, Ti and the like having an ionic radius smaller than Mn stabilize the second phase even after the LiMnO ₂ phase changes to the second phase and contribute to charge and discharge.

[0027]

Next, examples of the present invention will be described in detail together with comparative examples. Example 1 Mn oxide (Mn ₂ O ₃ : average particle diameter 46.
2 μm) and 4.03212% by weight of Cr oxide (Cr
₂ O ₃ : average particle size 20 μm) is 0.2111% by weight,
The oxide of n (SnO ₄ : average particle size 5 μm) is 0.251
1% by weight and then mixed, and the mixture was calcined at 800 ° C. for 10 hours in air to produce an intermediate. This intermediate is supplied to a reaction vessel made of PTFE, and then lithium hydroxide monohydrate (LiOH · H ₂ O) is added.
35 times the ratio of Li to Mn (95.507% by weight)
And subjected to a hydrothermal reaction under hydrothermal conditions of maintaining at a predetermined temperature of 230 ° C. for 6 hours. The reaction was further washed with water and dried under vacuum. Thereby, a positive electrode active material (LiSn _0.03 Cr _0.05 Mn _0.92 O ₂ ) was obtained. This active material was used as Example 1.

Example 2 3.9032% by weight of Mn oxide (Mn ₂ O ₃ : average particle size 46.2 μm) and 0% of Cr oxide (Cr ₂ O ₃ : average particle size 20 μm) .2043
% By weight and an oxide of Pb (PbO ₄ : average particle size 5 μm)
And 0.3858% by weight, respectively, and then mixed. The mixture was calcined at 800 ° C. for 10 hours in air to produce an intermediate. This intermediate is supplied to a PTFE reaction tank, and then lithium hydroxide monohydrate (LiOH
H ₂ O) by 35 times the ratio of Li to Mn (95.
5067% by weight) and subjected to a hydrothermal reaction under the same hydrothermal conditions as in Experimental Example 1. The reaction was further washed with water and dried under vacuum. Thereby, the positive electrode active material (Li
Pb _0.03 Cr _0.05 Mn _0.92 O ₂ ) was obtained. This active material was used as Example 2.

Comparative Example 1 An oxide of Mn (Mn ₂ O ₃ : average particle diameter 44.7 μm) was 5.101% by weight, and LiOH was used.
Weighing 94.899% by weight of H ₂ O (Mn
Li becomes 85 times equivalent to the above. ) And PT
It was supplied to a reaction vessel made of FE and subjected to a hydrothermal reaction under the same hydrothermal conditions as in Example 1. Next, the reaction product was washed with water and dried under vacuum to obtain a positive electrode active material. This positive electrode active material was used as Comparative Example 1.

<Comparative Tests and Evaluations> Table 1 shows the compounding ratios of the compounds for producing the positive electrode active materials of Examples 1 and 2 and Comparative Example 1, and the compositions of the produced positive electrode active materials. Further, a slurry was prepared by mixing the positive electrode active materials of Examples 1 and 2 and Comparative Example 1 with a binder and a conductive auxiliary, and the slurry was stretched on a positive electrode sheet by a doctor blade method and dried to obtain a slurry on the positive electrode current collector. The positive electrode sheets were laminated on each other to form a positive electrode. These positive electrodes were attached to a charge / discharge cycle test apparatus 21 as shown in FIG. In this device 21, an electrolyte solution 23 (a solution in which a lithium salt is dissolved in an organic solvent) is stored in a container 22, and the positive electrode 12 is connected to the electrolyte solution 23 together with the negative electrode 14 (metal lithium) and the reference electrode 24 (metal lithium). It is soaked, and the positive electrode 12, the negative electrode 14, and the reference electrode 24 are electrically connected to a potentiostat 25 (potentiometer). A charge / discharge cycle test was performed using this apparatus, and the discharge capacity of each positive electrode active material was measured. Table 2 shows the results when the current value during charging and discharging was 10 mA / g, and Table 3 shows the results when the current value during charging and discharging was 20 mA / g. Is set to 80 mA / g, and the results are shown in Table 4. FIG. 3 shows the X-ray diffraction patterns of the positive electrode active materials of Examples 1 and 2 and Comparative Example 1 (measured using Kα ray of copper).

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

Table 2 (Current value during charging and discharging: 10 mA / g)
As is clear from the above, in Comparative Example 1, when the charge and discharge were repeated, the discharge capacity gradually decreased, and when the charge and discharge were repeated six times, the discharge capacity was reduced by about 40% from the first time. In contrast, in Example 1, the decrease in discharge capacity was extremely small,
At the time of the repetition, it decreased only by about 32% from the first time. In Comparative Example 1, the first discharge capacity was 13
In contrast to 3.38 mAh / g, Examples 1 and 2
In this case, the first discharge capacity was 156.47 mAh / g and 1
77.05 mAh / g, about 17% and about 33%, respectively
Increased. Table 2 (Current value during charge / discharge: 10 mA /
g) and Table 3 (current value during charging / discharging: 20 mA / g), Example 1 has a charging / discharging current of 10 mA / g.
However, the capacity does not decrease as in Comparative Example 1 even when the capacity doubles to 20 mA / g. Thus, it can be said that the resistance of Example 1 is smaller than that of Comparative Example 1.

Table 3 (Current value during charging / discharging: 20 mA)
/ G), in Comparative Example 1, when the charge and discharge were repeated, the discharge capacity gradually decreased, and when the charge and discharge were repeated 19 times, the discharge capacity decreased by about 11% from the first time. On the other hand, in Example 1, the decrease in the discharge capacity was extremely small, and the decrease was only about 4% at the time of the 50th repetition. In Comparative Example 1, the first discharge capacity was 73.59 m.
Ah / g, whereas in Examples 1 and 2, the first discharge capacity was 138.80 mAh / g and 146.48.
mAh / g increased by about 89% and about 100%, respectively. As is clear from Table 4, the current value during charging and discharging could not be set to 80 mA / g in Comparative Example 1 and Example 2, whereas the current value during charging and discharging was changed in Example 1. Charge / discharge could be repeated 7 times while maintaining 80 mA / g. Further, as apparent from FIG. 3, it was found that the crystal systems of Examples 1 and 2 before the first charge were indexed as orthorhombic.

[0037]

As described above, according to the present invention, a part of Mn of the lithium manganate compound is converted into an element A (Sn, Pb) having an ionic radius larger than Mn and an element A having an ionic radius smaller than Mn. B is replaced with (Ti, Cr, Fe, Al, etc.) and, because it contains represented by composition formula _{(LiA x B y Mn 1-} xy O 2), the volume change due to charging and discharging cycle is small, That is, the phase change hardly occurs, and the second phase generated by the phase transformation also shows a charge / discharge reaction. As a result, even if the charging and discharging are repeated, there is no phase change of the orthorhombic phase, or even if there is, the spinel phase-like second phase generated by the phase change can be stabilized at a constant conversion rate. In addition, it is possible to suppress a decrease in charge / discharge capacity and volume energy density. The crystal system before the first charge is indexed by an orthorhombic system, or Sn or Pb is used as A, and Cr is used as B.
0.001 ≦ x ≦ 0.07, and 0.001
≦ y ≦ 0.1, and 0.02 ≦ (x + y) ≦ 0.17
If so, the above effects can be more remarkably exhibited.

Further, the oxide or acetate of Mn, the additive C and the additive D are weighed, respectively, or immediately, or either or both of the oxide or acetate of Mn and the additive C or D are subjected to a predetermined method. After heat treatment under the conditions to produce an intermediate, a predetermined amount of lithium hydroxide monohydrate is added to the mixture of the weighed substances, or the mixture of the weighed substance and the intermediate, or the intermediate, and the mixture is heated to a predetermined temperature. The reaction is carried out under a hydrothermal condition maintained for a predetermined time, and the reaction product is further washed and vacuum-dried, whereby the positive electrode active material can be obtained. Furthermore, when a lithium secondary battery is manufactured using the above-described positive electrode active material, generation of a spinel phase accompanying the progress of a charge / discharge cycle can be effectively prevented, so that a decrease in charge / discharge capacity can be suppressed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a main part of a lithium secondary battery according to an embodiment of the present invention.

FIG. 2 shows an apparatus used for a charge / discharge cycle test of the positive electrode active materials for lithium secondary batteries of Examples and Comparative Examples.

FIG. 3 is a diagram showing X-ray diffraction patterns of the positive electrode active materials of Examples 1 and 2 and Comparative Example 1.

FIG. 4 is an enlarged sectional explanatory view schematically showing a structure of a lithium secondary battery.

[Explanation of symbols]

 10. Lithium secondary battery

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tadashi Sugihara 1-297 Kitabukuro-cho, Omiya-shi, Saitama F-term in Mitsubishi Materials Corporation Research Laboratory 5H029 AJ05 AK03 AL12 AM02 AM07 5H050 AA07 BA16 BA17 CA09 CB12 EA24 FA19 GA02 GA27 HA01 HA02 HA14

Claims (5)

[Claims] 1. A lithium manganate compound, wherein M
Part of n is composed of an element A having an ionic radius larger than Mn and M
A positive electrode active material for a lithium secondary battery, comprising a composition represented by the following formula (1), which is substituted with an element B having an ionic radius smaller than n. _{LiA x B y Mn 1-xy} O 2 ...... (1) where, A is an element of one or both of Sn or Pb, B is selected from the group consisting of Ti, Cr, Fe and Al 1 Species or two or more elements, and 0 <x <0.
3, 0 <y <0.3, and 0 <(x + y) <
0.3. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the crystal system before the first charge is indexed by an orthorhombic system. 3. A is one or both elements of Sn and Pb, B is Cr, and 0.001 ≦ x ≦ 0.
07, 0.001 ≦ y ≦ 0.1, 0.02
The positive electrode active material for a lithium secondary battery according to claim 1, wherein ≤ (x + y) ≤ 0.17. 4. When the total weight of the objects to be weighed is 100% by weight, 2.9 to 5.3% by weight of an oxide or acetate of Mn;
Additive C comprised of one or both of Sn and Pb, or an oxide, hydroxide, chloride or acetate thereof, or a mixture thereof, in an amount of 0.01 to 1.3% by weight.
And one or more oxides, hydroxides or chlorides selected from the group consisting of Ti, Cr, Fe and Al, or an additive D0.
Weighing 15 to 0.3% by weight of each; 750 to 8 immediately after weighing, or the oxide or acetate of Mn and one or both of the additives C and D.
After baking in air at a predetermined temperature of 50 ° C. for 5 to 25 hours to produce an intermediate, the mixture of the weighed substances, or the mixture of the weighed substance and the intermediate, or the intermediate is charged with lithium hydroxide 1 The hydrate is converted to a 7 to 6 ratio of Li to Mn.
A step of adding the mixture so as to be 0 times and reacting under a hydrothermal condition of maintaining the mixture at a predetermined temperature of 140 to 280 ° C. for 2 to 30 hours; and a step of washing the reaction product with water or ethanol and then drying it in vacuo. A method for producing a positive electrode active material for a lithium secondary battery, comprising: 5. A lithium secondary battery manufactured using the positive electrode active material for a lithium secondary battery according to claim 1.