JP2002075365A - Positive electrode active material and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material consisting of lithium manganate material well balanced, having a high capacity, and excellent in the high-rate charge-discharge characteristics and cyclic characteristics and also provide a lithium secondary battery. SOLUTION: The positive electrode active material for lithium secondary battery consists of complex oxide of manganese having spinell type crystal structure expressed by Li1-xMn1-x-yMyO4, (provided that 0<=x<=0.4 and 0<=y<=0.15, and in the formula, M represents a metal or metals selected among Ni, Co, Cr, and Al), wherein a void is formed inside each particle of the positive electrode active material in such an arrangement that the ratio of the void section area to the particle section area lies within the range of 3.0-20%.

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 comprising a manganese composite oxide for a lithium secondary battery and a lithium secondary battery using the same.

[0002] The crystal structure is spinel, by the chemical formula _{Li 1-x Mn 1-xy} M y O 4 (0 ≦ x ≦ 0.4,0 ≦ y ≦
0.15, where M is at least one metal selected from the group consisting of Ni, Co, Cr and Al), the average particle size is in the range of 30 μm to 5 μm, and the critical area for BET is 0.5 m. / G to 2.5 m / g is a manganese composite oxide having a large amount of pores inside the particle.

[0003]

2. Description of the Related Art With the spread of devices such as mobile phones and personal computers, there is a strong demand for further reduction in size, weight and cost of lithium secondary batteries. In response to this demand, the development of lithium ion secondary batteries using lithium cobaltate represented by the chemical formula LiCoO ₂ for the positive electrode material and a carbon material capable of storing and releasing lithium ions for the negative electrode material has been promoted. Has been Also, the chemical formula L
Lithium nickelate represented by iNiO ₂ is also expected as a positive electrode material, and is being actively researched and developed.

However, there is a problem that cobalt as a raw material is expensive for LiCoO _{2, and} it is difficult to synthesize a material for LiNiO ₂ . In addition, both batteries have a problem in safety at the time of overcharging, and it is essential to incorporate a safety circuit into the battery pack, which hinders cost reduction of the batteries.

On the other hand, lithium manganate represented by the chemical formula LiMn ₂ O ₄ is LiCoO ₂ or LiNiO ₂
It is characterized by the fact that it is a material showing a battery voltage of 4V class, that it is easy to synthesize, and that manganese as a raw material is abundant and inexpensive as resources. In addition, from the viewpoint of the physical properties of the material, thermal stability is LiCoO ₂ or LiNi.
Since it is higher than O _2, and almost all of the lithium contained electrochemically is used for the charge / discharge reaction, even if overcharged, it is difficult for metallic lithium to precipitate on the negative electrode. It is highly safe and may not require installation of a protection circuit. from these things,
LiMn ₂ O ₄ is expected as a new active material for a lithium secondary battery replacing LiCoO ₂ and LiNiO ₂ .

A lithium secondary battery using this LiMn ₂ O ₄ as a positive electrode active material has a problem that the capacity is greatly deteriorated due to charge / discharge cycles. This may be caused by local collapse of the crystal structure due to expansion and contraction of the crystal lattice due to the charge / discharge reaction, or a decrease in conductivity due to finer particles, or a problem with conductive aids or current collectors such as acetylene black. Poor contact was considered.

As a method for solving this problem, a method has been proposed in which part of manganese is replaced with another metal element. For example, JP-A-3-219571 and JP-A-4
Japanese Patent Application Laid-Open No. 160760/1996 and Japanese Patent Application Laid-Open No. 8-217452 propose a material in which part of manganese is replaced with Co, Cr, Fe, and Ni. These proposals aim at reducing the lattice constant of the crystal by substituting a part of manganese with another metal element, and preventing the crystal from being destroyed by a charge / discharge cycle.

[0008]

However, with the spread of cordless devices, lithium secondary batteries are required to have higher capacity and higher rate characteristics. However, in the method disclosed in the above publication, by using lithium manganate in which a metal element is dissolved as a solid material as an active material, although cycle characteristics are improved, the Mn element involved in the electrochemical reaction is reduced. There is a problem that a reduction in capacity cannot be avoided.

On the other hand, it is conceivable to increase the thickness of the positive electrode active material applied to the current collector so as to increase the total amount of the active material that can be filled in the battery and prevent the battery capacity from decreasing. In such a case, the length of the electrode plate that can be inserted into the determined battery case is shortened, and the apparent current density is increased. As a result, there is a problem that high-rate charging and discharging cannot be performed.

[0010] Further, even if an active material having a higher specific surface area is simply obtained by miniaturization, the high-rate charge / discharge characteristics are not improved. It is considered that the reason for this is that the active material in the electrode plate is densely filled by miniaturization of the active material, so that ions cannot be sufficiently moved through the electrolytic solution.

As described above, although the cycle characteristics can be improved with lithium manganate having improved cycle characteristics by partially replacing manganese with another metal element, at the same time, the battery capacity is reduced or the high-rate charge / discharge characteristics are reduced. Has been invited.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object to provide a positive electrode active material and a lithium secondary battery comprising a lithium manganate material having a high capacity and a high balance of charge / discharge characteristics and cycle characteristics and being well balanced. With the goal.

[0013]

As a result of investigations aimed at solving the above problems, a new positive electrode active material comprising lithium manganate was developed. Specifically, the chemical formula L
_{i 1-x Mn 1-xy} M y O 4 (0 ≦ x ≦ 0.4,0 ≦ y ≦
0.15, where M is a positive electrode active material composed of a manganese composite oxide represented by the following formula: M is at least one metal selected from Ni, Co, Cr, and Al. As described above, in the conventional example, as shown in FIG. 1B, the particles A of the positive electrode active material were in a solid state, whereas the holes A were formed inside the particles A of the positive electrode active material. The ratio of the cross-sectional area of the pores H to the cross-sectional area of the particles A is 3.0%.
Something in the range of ~ 20% was found. By using the particles A of the positive electrode active material in a lithium secondary battery, a high-capacity battery excellent in cycle characteristics and rate characteristics was completed.

To explain in detail, lithium manganate is generally obtained as follows. Chemical formula MnC
The aqueous solution of sulfuric acid in which manganese ore represented by O ₃ is dissolved is neutralized with sodium hydroxide to precipitate manganese hydroxide represented by the chemical formula Mn (OH) ₂ . Furthermore, manganese oxide is obtained by oxidizing the precipitated manganese hydroxide, and the final product can be adjusted to manganese dioxide, dimanganese trioxide, and trimanganese tetroxide depending on the oxidation conditions.

The properties of the obtained manganese hydroxide, such as the particle size and density, are determined by the neutralization conditions at this time, specifically, the pH of the neutralization tank, the reaction temperature, and the additives. The properties of the lithium oxide will be determined. By adjusting the neutralization conditions, a large amount of pores can be provided inside the manganese hydroxide particles.

Manganese oxide is obtained from the manganese hydroxide thus obtained, and further obtained by the chemical formula Li _1-x Mn _1-xy M
_y O ₄ (0 ≦ x ≦ 0.4, 0 ≦ y ≦ 0.15, M in the formula
When lithium manganate represented by at least one metal selected from the group consisting of Ni, Co, Cr and Al) is synthesized, a material having a large amount of pores inside the particles can be obtained in the same manner as manganese hydroxide.

In the battery using this lithium manganate as a positive electrode active material, the cycle life is improved. We consider the reason as follows. Generally, when charging and discharging in a lithium secondary battery using lithium manganate for the positive electrode, the particles are refined due to the stress caused by the expansion and contraction of the positive electrode active material crystal lattice, causing a change in the conductive structure between the particles and a change in the active material. Poor contact between the current collector or active material and the conductive additive causes a reduction in cycle life. However, in the active material, it is considered that the pores formed in the particles absorb the stress applied to the particles, and the miniaturization as described above does not easily occur, and the cycle life deterioration is suppressed.

A battery using the above-mentioned lithium manganate as a positive electrode active material is characterized by having excellent high-rate discharge characteristics. This is because in the conventional active material, ions diffuse from the particle surface to the inside or from the particle inside to the surface in the solid, and a charge transfer reaction occurs at the interface with the electrolytic solution. However, since a large amount of pores are formed inside the active material, the electrolytic solution penetrates into the active material particles, the diffusion distance of ions is shorter than that of conventional materials, and the reaction surface area is smaller. The actual current density is reduced because of the widening. From the above points, it is considered that the lithium manganate of the present invention is excellent in high-rate discharge characteristics.

According to the study of the present inventor, the lithium secondary battery has a positive electrode made of an aluminum core material and a negative electrode made of a copper core material, which is made by applying a thin paste of an active material as a main material, removing a solvent, and removing the solvent. I am creating a board. The positive and negative electrodes are spirally formed with a separator interposed therebetween, and inserted into the battery case.
At this time, as the active material layer is thicker, the amount of active material that can be filled in a battery case of a fixed size increases, that is, the battery capacity increases, but when the active material layer is made thicker like this,
There is a disadvantage that the cycle life is shortened. The reason is that the thicker the positive electrode active material layer, the longer the diffusion distance of the electrolyte from the electrode plate surface to the inside becomes, and the diffusion speed cannot follow the charge / discharge reaction speed, and the reaction becomes uneven in the thickness direction of the electrode plate. The cause is considered to be the occurrence of Therefore, in an actual battery, a battery design must be made in consideration of a balance between cycle life characteristics and battery capacity. For this reason, in many cases, the thickness of the positive electrode plate is designed to be 250 μm or less. However, when the active material of the present invention is used in a battery, the active material itself holds a large amount of electrolyte in its pores, so that even if the active material layer is thickened, cycle deterioration can be suppressed to a small level.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the positive electrode active material of the present invention and a lithium secondary battery using the same will be described.

In FIG. 2, reference numeral 1 denotes a negative electrode which is formed by coating a negative electrode material on both sides of a copper foil, dried, rolled and cut into a predetermined size, and 2 denotes a nickel negative electrode lead spot-welded to the negative electrode 1. is there. No. 3 was prepared by mixing a conductive agent carbon black and a binder PVDF in a weight ratio of 100: 3: 4 to a positive electrode active material, applying the mixture on both sides of an aluminum foil, drying, rolling and cutting into a predetermined size. The positive electrode 4 is an aluminum positive electrode lead ultrasonically welded to the positive electrode 3. Reference numeral 5 denotes a separator made of a microporous film, which is interposed between cans of the negative electrode 1 and the positive electrode 3 and spirally wound to constitute an electrode group. Reference numeral 6 denotes an upper insulating plate made of propylene disposed above the electrode plate group, 7 denotes a lower insulating plate disposed below the electrode plate group, and 8 denotes a case housing the electrode plate, which is nickel-plated on iron. It is. Then, after the positive electrode lead 4 is spot-welded to the aluminum sealing plate 10 and the negative electrode lead 2 is spot-welded to the bottom of the case 8, a predetermined amount of a non-aqueous electrolyte is injected into the electrode group in the case 8, and the gasket 9 is removed. The completed battery is sealed by the sealing plate 10 through the intermediary. Reference numeral 11 denotes a positive electrode terminal of the battery provided on the outer surface of the sealing plate 10, and the case 8 also serves as a negative electrode terminal.

The cylindrical non-aqueous electrolyte secondary battery having such a structure was used as an evaluation battery, and more specifically, a prototype was produced according to the following procedure.

(Prototype Production Procedure 1) Chemical Formula L as Positive Electrode Active Material
_{i 1-x Mn 1-xy} M y O 4 at (x = 0.1, y = 0.05 ,
Acetylene black (AB) is 2.5% by weight based on the weight of the lithium manganate powder represented by M = Al).
Mix and add the binder polyvinylidene fluoride (PVD)
F) and N-methylpyrrolidone (NMP)
VDF is 3.0 to 6.0 for lithium manganate
% By weight and a solid content ratio of 50 to 70% by weight, kneading is performed for 90 minutes, and the viscosity is 15,000 to 250%.
A paste-like positive electrode paste of 00 cps was prepared.

(Trial Production Procedure 2) As the negative electrode active material, artificial graphite was used. After mixing the same binder as that of the positive electrode with respect to the weight of the artificial graphite so that PVDF becomes 9% by weight, NM
After adding P, kneading was performed to produce a paste-like negative electrode paste.

(Trial Production Procedure 3) A positive electrode paste having a thickness of 0.0
It was coated on both sides of a 2 mm aluminum foil, dried and rolled to obtain a positive electrode plate having a positive electrode mixture density of 2.8 g / cc and a width of 37 mm.
The electrode plate thickness and the electrode plate length were adjusted to predetermined values.

(Prototype Production Procedure 4) A negative electrode paste having a thickness of 0.0
A 14 mm copper foil was coated on both sides, dried and rolled to obtain a negative electrode plate. At this time, the coating amount of the negative electrode was adjusted such that the weight of the negative electrode active material per unit area was 3.5 times the weight of the corresponding positive electrode active material per unit area.

(Prototype 5) A lead made of aluminum was attached to the positive electrode plate, and a nickel lead was attached to the negative electrode plate. The thickness was 0.036 mm, the width was 40 mm, and the length was 1100 m.
The electrode plates were spirally wound through a separator made of polypropylene and polyethylene as main materials, and delivered to a battery case having a diameter of 17 mm and a height of 50 mm.

As the electrolytic solution, ethylene carbonate (E
C), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a mixed solvent of 1.5 mol / l
After dissolving LiPF ₆ in the above solution and injecting the solution, the cell was sealed to obtain a battery.

(Example 1) Six types of batteries having different porosity formed in the active material particles were trial-produced in accordance with the above-mentioned trial production procedures 1 to 5. The porosity formed inside the particles of the lithium manganate, a positive electrode active material used, was
(1) 1.0%, (2) 3.0%, (3) 7.0%, (4) 15
%, (5) 20%, and (6) 25%. At this time, the electrode plate thickness was 0.2 mm, and the electrode plate length was 360 mm.

(Example 2) The positive electrode paste adjusted according to the trial production procedure 1 with the porosity in the lithium manganate particles fixed at 7% was used, and the thickness of the electrode plate after drying was (1) 0.15 m
m, (2) 0.20 mm, (3) 0.25 mm, (4) 0.3
0 mm and (5) 0.35 mm. Thereafter, a battery was prototyped according to Procedures 2 to 5.

(Comparative Example 1) According to the above-mentioned trial production procedures 1 to 5, a battery was trial-produced using lithium manganate having less than 1.0% of voids in the particles and having almost no voids as the positive electrode active material. . At this time, the electrode plate thickness was 0.2 mm, and the electrode plate length was 360 mm.

(Comparative Example 2) A positive electrode paste prepared by preparing lithium manganate having less than 1.0% of pores in the particles with a positive electrode active material according to the trial production procedure 1 was used.
(1) 0.15mm, (2) 0.20mm, (3) 0.25m
m, (4) 0.30 mm, and (5) 0.35 mm. Thereafter, a battery was prototyped according to Procedures 2 to 5. The porosity was measured by solidifying the positive electrode plate with epoxy resin, polishing the cross section, removing the porosity due to grain boundaries by processing the image, and then measuring the ratio of active material to porosity. Was calculated.

These prototype batteries were subjected to an initial charge / discharge cycle of 4 cycles, stored for 7 days in a discharged state at 25 ° C., and then subjected to a charge / discharge cycle life test or a rate characteristic test.

Table 1 shows the results of the cycle characteristics evaluation of the batteries of Example 1 and Comparative Example 1 of the present invention for each three cells. The cycle characteristics were evaluated at 25 ° C. by charging and discharging at a constant current of 0.2 C with a charge upper limit voltage of 4.3 V and a discharge end voltage of 3.0 V. The results show the average value of the three evaluated cells.

[0035]

[Table 1] From the results in Table 1, it is clear that the batteries of Examples (2) to (5) are superior to the comparative example in the cycle characteristics. This is because, when the positive electrode plate of the cycled battery is observed with an SEM photograph, the positive electrode active material of the comparative example is finer, whereas such change is small in the electrode plate of the example. In the active material of the present invention, the cracking of the particles progresses due to the stress caused by the expansion and contraction of the manganate spinel in the discharge. It is considered that the reason for this is that there is little cracking of the material and that the pores formed in the active material contain the electrolyte and the charge / discharge reaction has progressed uniformly over the entire electrode plate. However, the reason why no effect was observed in Example (1) is considered that the number of pores formed in the active material particles was small and the effect was not exhibited. Further, in Example (6), it was found that the active material particles were pulverized during the rolling step when producing an electrode plate having many holes. In other words, it is considered that the pores in the particles are reduced, and a part of the active material is finely divided by the pulverization, so that the reaction becomes uneven and the cycle life is shortened.

Regarding the capacity in the first cycle, the batteries of Examples 1 (2) to (5) have a larger capacity than the comparative example. It is considered that the use efficiency of the active material is also increased because the pores in the particles contain the electrolytic solution.

From these results, it is found that the amount of vacancies in the active material is from 3% to 2% of the active material cross-sectional area.
It is preferably in the range of 0%.

Next, Table 2 shows the ratio of the discharge capacity discharged at a constant current of 2.0 C to the discharge capacity discharged at a constant current of 0.2 C for each battery in Example 2 and Comparative Example 2. In this rate characteristic evaluation, a constant current charge of 0.2 C with an upper limit voltage of 4.3 V was performed at 25 ° C. The discharge capacity was determined at a predetermined current value and a final voltage of 3.0 V.

[0039]

[Table 2] From Table 2, it can be seen that the rate characteristics of the examples (1) to (5) are improved as compared with the conventional examples (1) to (5).
When the discharge current increases, the diffusion rate of ions in the electrolytic solution cannot catch up with the reaction rate, and the discharge capacity decreases. However, if the contact area between the active material and the electrolyte increases, the rate characteristics can be improved. In the active material of the present invention, since the pores inside the particles can contain the electrolyte, it is considered that the reaction area is increased and the decrease in the discharge capacity at high rate discharge is suppressed to be smaller than that in the conventional example. .

In the battery using the positive electrode active material of Example 1 (6), the rate characteristics were also improved as compared with the conventional example in which the thickness of the positive electrode plate was changed. This is thought to be because the active material was pulverized and the pores were reduced by the rolling as described above, but the specific surface area was increased by the finer particles.

Next, Table 3 shows the first cycle capacity and cycle characteristics of Example 2 and Comparative Example 2. The cycle characteristics of these batteries were evaluated under the same conditions as described above.

[0042]

[Table 3] From Table 3, it can be seen that in the battery using the active material of the present invention, even if the active material layer of the positive electrode plate is thickened, the decrease in cycle life is smaller than in the conventional example. Especially the thickness of the electrode plate is 0.15m
There is almost no difference in cycle life between the electrode plates of m, 0.20 mm and 0.25 mm. On the other hand, in the battery of the comparative example, when the thickness of the positive electrode plate was increased, the cycle life was clearly shortened.

Experience has shown that the thickness of the electrode plate is related to the cycle life. This is considered to be because the thicker the active material layer is, the longer the diffusion distance of lithium ions is, and the reaction proceeds unevenly in the thickness direction of the electrode plate during charging and discharging. However, in the active material of the present invention, the pores formed in the active material particles contain the electrolytic solution,
It is considered that the diffusion of ions proceeds smoothly even when the electrode plate becomes thick.

By increasing the thickness of the active material layer, a battery case of a predetermined size can be filled with a large amount of active material, so that there is an advantage that a battery having a large capacity can be manufactured.

As described above, when the lithium manganate active material of the present invention is used, a large-capacity battery having excellent cycle characteristics and rate characteristics can be obtained.

[0046]

According to the present invention, as described above, pores are formed inside the particles of the positive electrode active material, and the amount of the pores is adjusted so that the ratio of the cross-sectional area of the pores to the particle cross-sectional area is 3.0% to 20%. Therefore, a positive electrode plate for a lithium ion secondary battery having a high liquid retention property and a small destruction of active material particles due to a charge / discharge reaction can be obtained. A secondary battery can be obtained.

[Brief description of the drawings]

1 shows particles of a positive electrode active material, (a) is a schematic view of particles of a positive electrode active material in the present invention, and (b) is a schematic view of particles of a conventional positive electrode active material.

FIG. 2 is a longitudinal sectional view of a lithium secondary battery.

[Explanation of symbols]

 A Particles of positive electrode active material H Vacancies

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takaya Saito 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Industry Co., Ltd. F-term (reference) 5H029 AJ02 AJ03 AJ05 AK03 AL07 AM03 AM05 AM07 DJ14 DJ16 DJ17 HJ02 HJ06 HJ07 HJ09 5H050 AA02 AA07 AA08 BA17 CA09 FA15 FA17 FA19 HA02 HA06 HA07 HA09

Claims (2)

[Claims] 1. The chemical formula Li having a crystal structure of spinel type
_1-XMn_1-xyM_yO _Four(0 ≦ x ≦ 0.4, 0 ≦ y ≦
0.15, where M is selected from Ni, Co, Cr, and Al
Manganese complex represented by at least one or more metals)
Positive electrode active material for lithium secondary batteries composed of composite oxide
Thus, pores are formed inside the particles of the positive electrode active material,
The amount is the ratio of the cross-sectional area of the pores to the cross-sectional area of the particles.
Positive electrode active material in the range of 0% to 20%
quality. 2. The chemical formula Li having a spinel type crystal structure.
_1-xMn_1-xyM_yO _Four(0 ≦ x ≦ 0.4, 0 ≦ y ≦
0.15, where M is selected from Ni, Co, Cr, and Al
Manganese complex represented by at least one or more metals)
Voids are formed in the composite oxide and inside the particles.
When the ratio of the cross-sectional area of the pores to the cross-sectional area of the particles is 3.0%.
Manganese composite oxide in the range of 20% to 20%
A lithium secondary battery characterized by being used as a lithium secondary battery.