JP2000040511A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the electrolyte retaining capacity of a positive electrode, and to suppress capacity drop in high temperature storage without dropping charge/discharge capacity at a room temperature by using the positive electrode containing an active material comprising a primary particle aggregate of LiMn2 O4 having pores between primary particles of a specified average size. SOLUTION: A positive electrode constituting a battery together with a negative electrode made of a carbonaceous material, a nonaqueous solvent electrolyte containing a lithium salt, and a separator is prepared by press molding a mixture of an active material of a primary particle aggregate of LiMn2O4 having a pore average size of 10 nm or less, a conductive auxiliary, and a binder. In charge/discharge, lithium ions are moved by solid diffusion within a crystal lattice of primary particles and liquid diffusion in an electrolyte retained between spaces of the aggregate. Permeation of the electrolyte into the active material caused by a capillary action is suppressed with increased in pore size, but not decreased in a pore size of 10 nm of less. The LiMn2O4 has a cubic system spinel single phase, and is obtained by mixing, baking and sieving powder of lithium hydroxide, lithium carbonate, and electrolytic manganese dioxide each having a given particle size.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery, and more particularly, to a lithium ion secondary battery having an improved positive electrode active material.

[0002]

2. Description of the Related Art With the rapid miniaturization of electronic equipment, there is an increasing demand for the development of a secondary battery that is small, lightweight, has a high energy density and can be repeatedly charged and discharged as its power source. As a secondary battery satisfying these requirements, a lithium secondary battery has attracted attention. In this lithium secondary battery, the potential of lithium as the negative electrode is extremely low,
Since the battery voltage is high and the volume and weight energy density of lithium are high, high energy density can be achieved.

As a conventional lithium secondary battery, a lithium secondary battery having a positive electrode containing an active material such as cobalt oxide, nickel oxide, and manganese oxide is known. In particular, L
Lithium manganese oxide having a spinel-type crystal structure represented by iMn ₂ O ₄ is environmentally harmless compared to other active materials, is abundant and inexpensive in terms of resources, and has high safety during overcharge. And other excellent properties.

However, a lithium secondary battery having a positive electrode containing a lithium manganese oxide having a spinel type crystal structure represented by LiMn ₂ O ₄ as an active material has been used in mobile phones and notebook personal computers. In addition to the battery capacity, there is a problem in terms of capacity deterioration, that is, cycle characteristics, in terms of use conditions and storage environment.

[0005] From this, a part of the 16d site of the spinel-type lithium manganese oxide is replaced with another transition metal,
For example, LiMn _2-x M substituted with Co, Ni, Fe, etc.
_{e x O 4 (Me = Co} , Ni, Fe) have been developed.

[0006]

As described above, when part of manganese in the spinel is replaced by another transition metal, the amount of manganese eluted by high-temperature storage decreases, but the charge capacity also decreases. .

The present invention provides a lithium ion secondary battery comprising a positive electrode containing lithium manganese oxide having a high electrolytic solution retention performance and capable of suppressing capacity deterioration due to high-temperature storage without a decrease in charge / discharge capacity at room temperature. It is something to offer.

[0008]

The lithium ion secondary battery according to the present invention is composed of an aggregate of primary particles of LiMn ₂ O ₄ and the average diameter of pores located between the primary particles is 10 nm or less. A positive electrode including a certain active material is provided.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a lithium ion secondary battery according to the present invention will be described in detail with reference to FIG.

For example, in a positive electrode can 1 made of stainless steel,
The positive electrode 2 is housed. Separator 3 is the positive electrode 2
Is located above. The separator 3 is impregnated with a non-aqueous electrolyte in which an electrolyte is dissolved in an organic solvent. The negative electrode 4 is disposed on the separator 3. At the opening of the positive electrode can 1, a negative electrode can 6 is provided via an insulating gasket 5, and the negative electrode can 6 and the positive electrode can 1 are provided.
The positive electrode 2, the separator 3 and the negative electrode 4 are hermetically sealed in the positive electrode can 1 and the negative electrode can 6 by caulking.

Next, the positive electrode 2, the negative electrode 4, the separator 3, and the non-aqueous electrolyte will be described in detail.

(1) Positive Electrode 2 The positive electrode 2 is composed of an aggregate of primary particles of LiMn ₂ O ₄ and the average diameter of pores located between the primary particles is 1
An active material having a thickness of 0 nm or less, a conductive additive such as graphite,
It is produced by pressure molding a mixture containing a binder such as polytetrafluoroethylene.

The primary particles of LiMn ₂ O ₄ preferably have an average particle size of 10 to 50 μm.

When the average diameter of the pores exceeds 10 nm,
The permeability of the electrolyte of the active material, which is an aggregate of the primary particles of LiMn ₂ O ₄ , is reduced, and it is difficult to increase the capacity. More preferably, the average diameter of the pores is 1 to 5 n.
m.

It is preferable that the mixing ratio of the positive electrode active material, the conductive additive, and the binder is 90: 7: 3 to 100: 10: 1.

(2) Negative Electrode 4 The negative electrode 4 is produced by pressure-forming a mixture comprising a carbonaceous material, a conductive agent and a binder.

As the carbonaceous material, for example, artificial graphite, natural graphite, pyrolytic carbon, coke, resin fired body, mesophase small sphere, mesophase pitch and the like can be used.

As the conductive material, for example, acetylene black, carbon black or the like can be used.

Examples of the binder include styrene-butadiene latex (SBR), carboxymethyl cellulose (CMC), and polytetrafluoroethylene (PTC).
FE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), nitrile-butadiene rubber (NBR), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetra Fluoroethylene terpolymer, polytrifluoroethylene (PTrF
E), a vinylidene fluoride-trifluoroethylene copolymer, a vinylidene fluoride-tetrafluoroethylene copolymer, or the like can be used.

(3) Separator 3 The separator 3 is made of, for example, a polypropylene nonwoven fabric, a microporous polyethylene film, or the like.

(4) Non-aqueous electrolyte This non-aqueous electrolyte has a composition in which an electrolyte is dissolved in a non-aqueous solvent.

Examples of the electrolyte include lithium borofluoride (LiBF ₄ ) and lithium hexafluorophosphate (Li
PF ₆ ), lithium perchlorate (LiClO ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium aluminum chloride (LiAlCl) Can be mentioned.

Examples of the nonaqueous solvent include ethylene carbonate, 2-methyltetrahydrofuran, 1,2
1 selected from dimethoxyethane, diethoxyethane, 1,3-dioxolan, and 1,3-dimethoxypropane
Species or mixtures of two or more can be mentioned.

The amount of the electrolyte dissolved in the non-aqueous solvent is as follows:
It is desirably 0.5 to 1.5 mol / l.

The present inventors have found that a positive electrode active material composed of an aggregate of primary particles of LiMn ₂ O ₄ and having an average diameter of pores located between the primary particles of 10 nm or less has excellent electrolytic solution retention performance. And found that a lithium ion secondary battery provided with a positive electrode containing this positive electrode active material can suppress capacity deterioration due to high-temperature storage without a decrease in charge / discharge capacity at room temperature.

That is, the aggregate (secondary particles) of the primary particles of LiMn ₂ O ₄ is incomplete but one LiMn ₂ O 4
_It is thought to be a _four- crystal structure particle, and lithium in the primary particle intercalates between layers in the crystal lattice due to charge and discharge, and moves by solid state diffusion in an ionic state. At this time, the space between the primary particles is involved in holding the electrolyte and the electrolyte solution, and these spaces are considered to be pores filled with the electrolyte. The movement of lithium ions during charge and discharge involves liquid diffusion in the space of the aggregate, but the speed of mass transfer is higher in liquid diffusion, and the resistance to the liquid has a large effect on the maximum diameter. Is easily estimated. Simply put, the larger the pore size, the faster the movement of the substance.

However, when the pore diameter is larger, the permeation of the electrolytic solution due to the capillary phenomenon is easily suppressed, and the amount of the electrolytic solution held by the secondary particles is reduced, so that the capacity is reduced.

From these facts, the present inventors have determined that L
An active material having an average pore diameter of 10 nm or less in an aggregate of primary particles of iMn ₂ O ₄ was determined to have high electrolytic solution retention performance, and a lithium ion secondary battery provided with a positive electrode containing this active material Found that capacity degradation due to high-temperature storage was suppressed without lowering the charge / discharge capacity at room temperature, and high performance could be achieved.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail.

(Examples 1 to 5 and Comparative Example 1) First, as shown in Table 1 below, a lithium hydroxide powder having an average particle diameter of 45 μm and a lithium carbonate powder were mixed at a lithium carbonate / total lithium ratio of 0,5. The mixed powder mixed to obtain 10, 40, and 80 and the electrolytic manganese dioxide powder having an average particle size of 25 μm were weighed so that the molar ratio became Li / Mn = 1.05 / 2, and the mixture was weighed using a ball mill. The mixed powder was prepared by mixing for hours. Subsequently, these mixed powders were fired in an air atmosphere at 800 ° C. for 48 hours and cooled. Subsequently, each fired powder was sieved to obtain a fired powder having an average particle size of 30 μm.

The pore size of the synthesized fired powder was measured.
The results are shown in Table 1 below. These calcined powders are X
DR confirmed that it was a cubic spinel single phase.

Next, the calcined powder, carbon black as a conductive material, and polytetrafluoroethylene as a binder were uniformly mixed at a weight ratio of 100: 10: 5, and then pressure-molded. 0.4mm thick, 1 diameter
Six types of 6.0 mm positive electrode pellets were prepared.

A negative electrode having a thickness of 1.20 mm and a diameter of 16.0 mm was prepared by punching out lithium metal.

Next, each positive electrode mixture is accommodated in a positive electrode can made of stainless steel, and the negative electrode is accommodated in a negative electrode can made of stainless steel. A separator made of a nonwoven polypropylene fabric is placed between the positive electrode mixture and the negative electrode. Six coin-type lithium secondary batteries having the structure shown in FIG. 1 described above were arranged and assembled.

As an electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was mixed with a mixed solvent of ethylene carbonate and methyl ethyl carbonate (a mixed volume ratio of 2:
A composition having a composition of 1.0 mol / l dissolved in 1) was used.

The obtained secondary batteries of Examples 1 to 5 and Comparative Example 1 were stored in a thermostat at 60 ° C. for one week, and the remaining capacity (C) after high-temperature storage was determined according to the following equation. The results are shown in Table 1 below.

C (%) = C _after / C _{before × 100} where C _before : the discharge capacity of the battery at room temperature _before storage at high temperature, and C _after : the discharge capacity of the battery at room temperature _after storage at high temperature.

[0038]

[Table 1]

As is clear from Table 1 above, the pore diameter was 10
The secondary batteries of Examples 1 to 5 provided with a positive electrode including, as an active material, an aggregate of primary particles of LiMn ₂ O ₄ having a diameter of not more than 10 nm is an aggregate of primary particles of LiMn ₂ O ₄ having a pore diameter exceeding 10 nm. It can be seen that the residual capacity after high-temperature storage shows a high value of 70 to 80% as compared with Comparative Example 1 provided with a positive electrode containing as an active material.

(Examples 6 to 9) First, lithium carbonate powder and electrolytic manganese dioxide powder having average particle diameters of 11 μm, 16 μm, 25 μm, and 41 μm were used in a molar ratio of Li / Mn = 1.
Weighed so as to become 05/2, and weighed 24 using a ball mill.
The mixed powder was prepared by mixing for hours. Subsequently, these mixed powders are fired in an air atmosphere at 800 ° C. for 48 hours,
Cool. Subsequently, each fired powder was pulverized to obtain a fired powder having an average particle size of 10 μm.

The pore size of each of the calcined powders was measured.
The results are shown in Table 2 below. These calcined powders are X
DR confirmed that it was a cubic spinel single phase.

Four types of coin-type lithium secondary batteries were assembled in the same manner as in Example 1 except that the above-mentioned fired powder was used and the positive electrode mixture produced in the same manner as in Example 1 was used.

The obtained secondary batteries of Examples 6 to 9 were stored in a thermostat at 60 ° C. for one week, and the remaining capacity (C) after high-temperature storage was determined in the same manner as in Example 1. The results are shown in Table 2 below.

[0044]

[Table 2]

As is apparent from Table 2, the pore diameter is 10
The secondary batteries of Examples 6 to 9 each including a positive electrode including, as an active material, an aggregate of primary particles of LiMn ₂ O ₄ having a size of 10 nm or less exhibit a high residual capacity after storage at high temperature of 70 to 80%. You can see that.

(Example 10 and Comparative Examples 2 to 4)
Lithium carbonate powder and electrolytic manganese dioxide powder having an average particle diameter of 30 μm were weighed so that the molar ratio became Li / Mn = 1.05 / 2, and mixed using a ball mill for 24 hours to prepare a mixed powder. Subsequently, these mixed powders were fired in an air atmosphere at 600 ° C., 650 ° C., 700 ° C., and 750 ° C. for 48 hours and cooled. Subsequently, each fired powder was pulverized to obtain a fired powder having an average particle size of 10 μm.

The pore size of each of the obtained calcined powders was measured.
The results are shown in Table 3 below. These calcined powders are X
DR confirmed that it was a cubic spinel single phase.

Four types of coin-type lithium secondary batteries were assembled in the same manner as in Example 1, except that the above-described fired powders were used and the positive electrode mixture produced in the same manner as in Example 1 was used.

The obtained secondary batteries of Example 10 and Comparative Examples 2 to 4 were stored in a thermostat at 60 ° C. for one week, and the remaining capacity (C) after high-temperature storage was determined as in Example 1. .
The results are shown in Table 3 below.

[0050]

[Table 3]

As is clear from Table 3, the pore diameter is 10
The secondary battery of Example 10 provided with a positive electrode including, as an active material, an aggregate of primary particles of LiMn ₂ O ₄ having a particle size of 0.1 nm or less,
It can be seen that the residual capacity after high-temperature storage shows a higher value than Comparative Examples 2 to 4 each having a positive electrode containing an aggregate of primary particles of LiMn ₂ O ₄ having a pore diameter exceeding 10 nm as an active material. However, in Example 10, since the temperature (sintering temperature) at the time of synthesizing the fired powder was lower than in Examples 1 to 9, the remaining capacity was lower than in the secondary batteries of those examples.

[0052]

As described above in detail, according to the present invention, a positive electrode containing a lithium manganese oxide having a high electrolyte retention performance is provided, and the capacity deterioration due to high-temperature storage without lowering the charge / discharge capacity at room temperature. It is possible to provide a high-performance lithium-ion secondary battery with reduced power consumption.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a button-type lithium ion secondary battery according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Positive electrode can, 2 ... Positive electrode, 3 ... Separator, 4 ... Negative electrode, 6 ... Negative electrode can,

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H003 AA03 AA04 BB05 BC01 BC04 BD00 5H014 AA01 BB05 BB06 CC07 EE10 HH02 5H029 AJ04 AK03 AL06 AL07 AM03 AM04 AM07 DJ13 DJ16 HJ06 HJ09

Claims (1)

[Claims] 2. The method according to claim 1, wherein the primary particles are composed of an aggregate of primary particles of LiMn ₂ O ₄ , and the average diameter of pores located between the primary particles is 1
A lithium ion secondary battery including a positive electrode including an active material having a thickness of 0 nm or less.