JP2002025527A - Nonaqueous electrolytic secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in discharging capacity caused by repetition of charging and discharging in a nonaqueous electrolytic secondary battery using manganese-containing complex oxide as a positive electrode active substance. SOLUTION: The cycle life of a battery can be significantly improved by a separator surface modified by a cation exchange group such as carboxylic acid group and sulfonic group or the like. This is considered that because manganese is suppressed from adhesion to or deposition on a negative electrode surface by the fact that the manganese ions eluted into an electrolytic solution accompanied with a repetition of charging and discharging is made to react with a cation exchange group and trapped on the separator surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in charge / discharge cycle life of a nonaqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, as global environmental problems have become more serious,
It is planned to improve the energy efficiency and environmental compatibility of society as a whole by utilizing high energy density secondary batteries. In particular, in the fields of electric vehicles, hybrid vehicles, and power storage systems, application of non-aqueous electrolyte secondary batteries using lithium transition metal composite oxide for the positive electrode and carbonaceous material for the negative electrode as batteries that meet the required performance , And research and development are underway to increase its size.

As a positive electrode active material for a non-aqueous electrolyte secondary battery, a lithium manganese composite oxide which has a particularly large amount of resources and can be supplied at low cost is promising. However, the battery using the lithium manganese composite oxide has a problem that the capacity is greatly reduced due to the charge / discharge cycle, and the capacity is particularly remarkable under a high temperature condition.

[0004]

As described above, the conventional technique has a problem that the cycle life performance of a nonaqueous electrolyte secondary battery using a manganese-containing composite oxide as a positive electrode active material is insufficient. there were. In the future, in order to expand applications of nonaqueous electrolyte secondary batteries, further improvement in cycle life performance is indispensable.

It has been reported that a manganese-containing composite oxide reacts with hydrofluoric acid (HF) in an electrolytic solution to elute manganese. The reason why the capacity of a non-aqueous electrolyte secondary battery using a manganese-containing composite oxide as a positive electrode active material is large is that manganese in the active material elutes into the electrolytic solution with repeated charge and discharge and adheres to the negative electrode surface. Alternatively, it is considered that the charge / discharge reaction is hindered by the precipitation.

To improve the performance of a positive electrode using a manganese-containing composite oxide as the positive electrode active material, a manganese composite oxide (LiMn ₂ O ₄ ) having a spinel structure is used to control the crystal structure, Means of substituting a part of the metal with another element have been proposed, and although a certain effect has been reported at present, it is still insufficient to be applied to a practical battery in terms of cycle life performance.

[0007]

SUMMARY OF THE INVENTION The present invention aims at minimizing the decrease in capacity due to repetition of charge / discharge by suppressing the attachment / precipitation of manganese eluted from the positive electrode active material to the surface of the negative electrode plate. Therefore, the surface modification for imparting a cation exchange group to the separator is performed, and the eluted manganese ions react with the cation exchange group to trap manganese on the surface of the separator or in the pores. The present invention provides a positive electrode containing a manganese-containing composite oxide as an active material, a negative electrode containing a material capable of occluding and releasing lithium as an active material, a separator mainly composed of a microporous polyolefin resin film, and a non-aqueous electrolyte. Wherein the surface of the separator is modified with a cation exchange group.

The manganese-containing composite oxide is a lithium manganese composite oxide having a spinel structure. Further, as a cation exchange group, a carboxylic acid group (—COO)
H) or those obtained by replacing hydrogen of a carboxylic acid group with another cation (—COOLi, —COONa, —COOK)
), Or a sulfonic group (—SO ₃ H) or a substance obtained by substituting hydrogen of a sulfonic group with another cation (—SO ₃ L
i, -SO ₃ Na, is characterized by the use of -SO ₃ K, etc.). Further, the ion exchange capacity of these cation exchange groups is preferably in the range of 0.1 to 1.0 meq / g. The ion exchange capacity is a numerical value representing the amount of ions adsorbed by 1 g of the ion adsorbent (separator in this case) in milligram equivalent.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a method of modifying a carboxylic acid group on the surface of a separator, a method of imparting a functional group (-COOH) of acrylic acid (CH ₂ = CHCOOH) to the surface by performing an acrylic acid graft polymerization treatment. is there. The acrylic acid graft polymerization treatment involves immersing the nonwoven fabric in an aqueous solution of acrylic acid (vinyl monomer) and a polymerization initiator and irradiating the ultraviolet light in a nitrogen atmosphere. Can be controlled.

As a method for modifying the surface of the separator with a sulfone group, for example, there is a method in which a separator formed by stretching a polyethylene molded film is immersed in fuming sulfuric acid to give a sulfone group to the surface. Here, by changing the immersion time in fuming sulfuric acid, the amount of sulfone groups to be provided can be controlled.

As a method for replacing the hydrogen ion of the carboxylic acid group or the sulfone group with another cation, for example, a separator provided with the carboxylic acid group or the sulfone group by the above-mentioned method may be used in an aqueous solution containing the cation. There is a method of immersion.

[0012] The positive electrode plate and the negative electrode plate are wound through the separator surface-modified with the cation exchange group in this manner to form an electrode body. The electrode body is housed in a battery can, a non-aqueous electrolyte is injected, and then sealed to produce a non-aqueous electrolyte secondary battery. The procedure for producing the battery is no different from the usual method.

The present invention is aimed at suppressing the adhesion and precipitation of manganese eluted from the positive electrode active material to the surface of the negative electrode plate. As the positive electrode active material, lithium manganese composite oxide Li having a spinel structure is used. _1-x Mn ₂ O ₄ (0 ≦ x <
In addition to 1), a lithium-manganese composite oxide Li _1-x MnO ₂ (0 ≦ x <
1) and those obtained by substituting a part of manganese with another transition metal in the spinel-type lithium manganese composite oxide, for example, Li _1-x Mn _2-y Co _y O ₄ (0 ≦ x <1, 0 <y ≤
0.3), which part of the transition metal was replaced with manganese in the lithium transition metal composite oxides such as lithium nickel composite oxide, for _{example, Li 1-x Ni 1-} y Mn y O 2 (0
≦ x <1, 0 <y ≦ 0.3) and the like. Further, a positive electrode active material obtained by mixing these may be used. And
When a granular active material is used, for example, a positive electrode plate can be manufactured by forming a mixture including active material particles, a conductive additive, and a binder on a metal current collector such as aluminum.

As the negative electrode active material, Al, Si, Pb,
Alloy of Sn, Zn, Cd, etc. with lithium, LiFe
₂ O _3, WO _2, transition metal oxides such as MoO _2, graphite, carbonaceous materials such as carbon, Li ₅ (Li ₃ N) lithium nitride, or the like, or a metal lithium foil, or even a mixture thereof Good. When a granular carbonaceous material is used, for example, a negative electrode plate can be manufactured by forming a mixture comprising active material particles and a binder on a metal current collector such as copper.

As the separator, a separator composed of a microporous film of a polyolefin resin such as polyethylene or polypropylene, a laminate of two or more of these microporous films, or a nonwoven fabric or a fluororesin is added to these microporous films. Those in which microporous membranes are laminated can be used. further,
Even if one polyethylene is taken, any polyethylene such as various types of high-density, medium-density, and low-density branched polyethylenes, linear polyethylenes, high-molecular-weight and ultra-high-molecular-weight polyethylenes can be used. In addition, those containing appropriate amounts of additives such as various plasticizers, antioxidants, and flame retardants may be used.

The cation exchange group modifying the surface of the separator includes a carboxylic acid group (—COOH) and a hydrogen ion of the carboxylic acid group substituted with another cation, a sulfone group (—SO ₃ H) and In addition to those in which the hydrogen ion of the sulfone group is substituted by another cation, those in which the hydrogen ion of the phosphon group (—PO (OH) ₂ ) and the phosphon group are substituted by another cation are also effective and can be used.

As the electrolyte, an inorganic solid electrolyte, a polymer solid electrolyte, an electrolytic solution and the like can be used. When a non-aqueous electrolyte lithium secondary battery is manufactured, for example, ethylene carbonate, propylene carbonate is used as the electrolytic solvent. Dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2
Polar solvents such as diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, or mixtures thereof;

Lithium salts dissolved in these electrolyte solvents include LiPF ₆ , LiClO ₄ , LiBF
₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , L
iN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ ,
LiN (COCF ₃ ) ₂ and LiN (COCF ₂ CF ₃ )
Salts such as ₂ , or mixtures thereof, can be used.

The shape of the battery is not particularly limited, and the present invention is applicable to non-aqueous electrolyte batteries having various shapes such as a square, cylindrical, long cylindrical, coin, button, and sheet batteries. Applicable to secondary batteries.

[0020]

Embodiments of the present invention will be specifically described below.

(Positive Electrode Plate) The positive electrode plate was manufactured as follows. That is, 90 parts by weight of a spinel-type lithium manganese composite oxide (LiMn ₂ O ₄ ) as an active material, 5 parts by weight of polyvinylidene fluoride as a binder, and 5 parts by weight of acetylene black as a conductive material were mixed, and mixed with the solvent. N-methyl-2-pyrrolidone was added and kneaded to form a paste. This paste was applied to a current collector made of an aluminum foil having a thickness of 20 μm, dried, pressed, and then cut into predetermined dimensions to produce the paste.

(Negative Electrode Plate) On the other hand, the negative electrode plate is prepared by mixing 90 parts by weight of a graphite powder of a carbonaceous material and 10 parts by weight of polyvinylidene fluoride as a binder, and adding N-
Methyl-2-pyrrolidene was added and kneaded to form a paste. This paste was applied to a current collector made of a copper foil having a thickness of 12 μm, dried, pressed, and then cut into a predetermined size to produce the paste.

(Separator) The separator is a microporous film obtained by uniaxially stretching a molded film made of polyethylene resin, and has a weight average molecular weight of 1.5 million and a porosity of 50.
%, And a thickness of 30 μm. This separator is referred to as Comparative Example 1 separator.

Acrylic Acid Graft Polymerization Treatment on Separator Acrylic acid graft polymerization treatment is performed on a separator formed by stretching a polyethylene molded film, and a functional group of acrylic acid (CH ₂ ＣＨCHCOOH) (—COO
H). Here, the acrylic acid graft polymerization treatment is performed by immersing the nonwoven fabric in an aqueous solution of acrylic acid (vinyl monomer) and a polymerization initiator and irradiating the nonwoven fabric with ultraviolet rays in a nitrogen atmosphere. By changing the ultraviolet irradiation time, the ion exchange capacity becomes 0.03, 0.1
Separators of 0, 0.50, 1.00 and 1.50 meq / g were made.

The ion exchange capacity was measured by a potassium determination method. This measurement was performed according to the following procedure. The sample was immersed in 1.0 M hydrochloric acid (temperature 60 ° C.), washed with water, and then 0.1 M potassium hydroxide solution (temperature 60 ° C.)
Immersed for 2 hours. (By this operation, the functional group -CO
OH adsorbs potassium ion by an ion exchange reaction and changes to -COOK. Next, the amount of potassium ions remaining in the solution without being adsorbed was quantified by adding hydrochloric acid and phenolphthalein dropwise. Finally, the gram equivalent of the potassium ion adsorbed per 1 g of the sample was calculated to determine the ion exchange capacity.

The separators thus provided with acrylic acid functional groups were used as separators of Comparative Example 2, Example 1, Example 2, Example 3 and Comparative Example 3 in ascending order of ion exchange capacity.

L of the carboxylic acid group provided to the separator
i-Ion Substitution Treatment- The separator of Example 3 obtained by the above-mentioned acrylic acid graft polymerization treatment was immersed in 1.0 M hydrochloric acid (temperature 60 ° C.), washed with water, and then 0.1 M lithium hydroxide solution (temperature 60 ° C.). C) for 2 hours to replace H ions of the carboxylic acid groups with Li ions. The separator subjected to such treatment was used as the separator of Example 4.

-Sulfonation treatment of separator- A separator formed by stretching a polyethylene molded film was immersed in fuming sulfuric acid to give a sulfone group on the surface. By changing the immersion time, the ion exchange capacity becomes 0.03,
0.10, 0.50, 1.00 and 1.50 meq /
g of separator was prepared. In this case, the ion exchange capacity was also determined by the above-described potassium determination method. The separators thus provided with the sulfone groups were prepared in the order of Comparative Example 4, Example 5, Example 6,
The separators of Example 7 and Comparative Example 5 were used.

The Li of the sulfone group added to the separator
Ion Substitution Treatment- The separator of Example 7 obtained by the above sulfonation treatment was immersed in 1.0 M hydrochloric acid (temperature 60 ° C.), washed with water, and then placed in a 0.1 M lithium hydroxide solution (temperature 60 ° C.). For 2 hours to replace H ions of the sulfone groups with Li ions. The separator subjected to such a treatment was referred to as the separator of Example 8.

(Battery) The above-described positive electrode plate and negative electrode plate were stacked with a separator interposed therebetween, and spirally wound to form an electrode body. The electrode body was housed in a bottomed cylindrical stainless steel container having a height of 65 mm and a diameter of 18 mm, a lead wire was welded to the positive electrode and the negative electrode terminal, and a lid was attached. The designed capacity was 1200 mA.
h sealed battery. In the non-aqueous electrolyte, 1 mol / l of lithium hexafluorophosphate was added to an organic solvent in which ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) were mixed at a ratio of 2: 1: 2. Was used.

(Charge / Discharge Cycle Test) The charge / discharge cycle test of the battery was performed under the following conditions. Charging: Charging at a constant current of 1 A, and after reaching 4.1 V, charging at a constant voltage of 4.1 V for a total of 3 hours. Discharge: Discharged at a constant current of 1A, reached 2.7V and terminated. The ambient temperature was 25 ° C.

(Test Results) The acrylic acid graft polymerization-treated separators of Examples 1 to 5, Examples 6 and 7
FIG. 1 shows the results of charge / discharge cycle tests of various batteries prototyped using the separator in which the H ion of the carboxylic acid group was replaced with Li ion and the separator of Comparative Example.

In the case where the separator of the comparative example was used, the discharge capacity was greatly reduced with the progress of the charge / discharge cycle.
At about 0 cycles, the capacity dropped to 700 mAh or less.
On the other hand, when a separator subjected to acrylic acid graft polymerization was used, the ion exchange capacity was 0.1 meq / g.
As described above, the reduction in the discharge capacity due to the progress of the charge / discharge cycle was reduced, and the effect of significantly improving the cycle life performance was observed. In addition, as the ion exchange capacity is increased, the cycle life performance tends to be improved. However, even if the ion exchange capacity is larger than 1.0 meq / g, the cycle life performance is improved more than the case of the ion exchange capacity of 1.0 meq / g. No effect was seen. Therefore, when performing the acrylic acid graft polymerization treatment, the ion exchange capacity is 0.1 to 1.0 me.
It is preferably set to q / g.

Next, FIG. 2 shows the results of charge / discharge cycle tests of various batteries prototyped using the sulfonated separators of Examples 6 to 10 and the comparative separator.

Also in the case of the separator subjected to the sulfonation treatment, the ion exchange capacity was 0.1 meq / g or more, and the effect of improving the cycle life performance was observed. As the ion exchange capacity is increased, the cycle life performance tends to be improved.
Even if it is larger than 0 meq / g, ion exchange capacity is 1.0
No improvement effect more than the case of meq / g was observed. Therefore, also in the case of performing the sulfonation treatment, the ion exchange capacity is preferably set to 0.1 to 1.0 meq / g.

After the charge / discharge cycle test, the test battery was disassembled, and the surfaces of the positive electrode plate, the negative electrode plate, their current collectors, connection leads, separators, battery containers and lids were observed with a magnifying glass. No abnormality was particularly found in the case of using the separator provided with the sulfone group.

As described above, in a nonaqueous electrolyte secondary battery using a manganese-containing composite oxide as a positive electrode active material, the cycle of the battery is modified by modifying the separator surface with a small amount of cation exchange groups. The life performance was significantly improved. The manganese-containing composite oxide reacts with hydrofluoric acid (HF) in the electrolytic solution and elutes manganese, so that the eluted manganese adheres and precipitates on the surface of the negative electrode, causing deterioration of battery performance. Conceivable.

In the present invention, the deterioration of the performance is suppressed by modifying the separator surface with a very small amount of cation exchange group. It is presumed that it is not possible to reach the negative electrode surface by being trapped on the surface or in the pores, and as a result, the deterioration of the negative electrode performance is suppressed.

In any case, the separator of the present invention
It has the excellent effect of improving the performance of lithium-ion batteries, and unlike the method of adding an ion-exchange resin, does not occupy the space inside the battery. However, it is suitable.

In the above examples, the effect of improving the battery performance of the separator subjected to the acrylic acid graft polymerization treatment and the separator of the single layer subjected to the sulfonation treatment was verified. The separator may be combined, laminated and integrated.
Further, separators having different ion exchange capacities may be combined, stacked, and integrated to form a separator.
Further, the above surface modification treatment may be performed before stretching the resin film to form a porous film.

In the above examples, the conditions for the acrylic acid graft polymerization treatment and the conditions for the sulfonation treatment were exemplified. However, if an appropriate amount of cation exchange groups can be modified on the separator surface, these conditions should be modified. In addition, it is also possible to apply. Further, if there is another appropriate method for providing a carboxylic acid group, that method may be used.

[0042]

According to the present invention, the cycle life performance of a non-aqueous electrolyte secondary battery using a manganese-containing composite oxide as a positive electrode active material is improved by modifying the separator surface with a small amount of cation exchange groups. Is remarkably improved. The present invention does not occupy the space in the battery unlike the method of adding an ion exchange resin or the like, and is therefore suitable for designing a battery with a higher capacity.

[Brief description of the drawings]

FIG. 1 is a diagram showing charge / discharge cycle characteristics of a non-aqueous electrolyte secondary battery using a separator subjected to an acrylic acid graft polymerization treatment.

FIG. 2 is a diagram showing charge / discharge cycle characteristics of a nonaqueous electrolyte secondary battery using a sulfonated separator.

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Claims (5)

[Claims] 1. A positive electrode containing a manganese-containing composite oxide as an active material, a negative electrode containing a material capable of occluding and releasing lithium as an active material, a separator mainly composed of a microporous polyolefin resin membrane, and a non-aqueous electrolyte. A non-aqueous electrolyte secondary battery comprising a liquid, wherein the surface of the separator is modified with a cation exchange group. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the manganese-containing composite oxide is a lithium manganese composite oxide having a spinel structure. 3. The nonaqueous electrolyte secondary solution according to claim 1, wherein the cation exchange group is a carboxylic acid group or a hydrogen ion of a carboxylic acid group substituted with another cation. battery. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the cation exchange group is a sulfone group or a hydrogen ion of the sulfone group substituted with another cation. 5. The ion exchange capacity of the cation exchange group is:
The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the non-aqueous electrolyte secondary battery has a concentration of 0.1 to 1.0 meq / g.