JPH09171825A - Secondary battery with non-aqueous solvent electrolyte - Google Patents

Abstract

(57) Abstract: A lithium secondary battery having a high energy density, capable of rapid charging with a large current discharge, a long charge / discharge life, and a low cost is provided. A negative electrode capable of charging and discharging lithium ions,
In a secondary battery having a positive electrode capable of reversibly electrochemically reacting with lithium ions, and an electrolytic solution in which an ion dissociative lithium salt is dissolved in a non-aqueous solvent, dimethyl carbonate and ethylene carbonate are used as the non-aqueous solvent of the electrolytic solution. And a secondary battery using a mixed solvent containing methyl acetate. It is preferable that the non-aqueous solvent contains 10 to 60 ethylene carbonate, 20 to 60 dimethyl carbonate, and 5 to 40% by volume of methyl acetate.

Description

Detailed Description of the Invention

[0001]

The present invention particularly relates to a secondary battery having a non-aqueous solvent electrolyte having a high voltage, a high energy density, and a large charge / discharge capacity.

[0002]

2. Description of the Related Art As portable electronic devices have become smaller and lighter, (1) they have a high voltage and a high energy density as their power sources, and (2) they can be rapidly charged with a large current discharge due to their intended use.
There is a demand for (3) a long charge / discharge life and (4) an inexpensive secondary battery. As a battery that meets such demands, development of a high-performance secondary battery having a negative electrode capable of charging and discharging lithium ions and a positive electrode capable of charging and discharging lithium ions, that is, a lithium secondary battery is expected. The nickel-cadmium batteries, nickel-hydrogen batteries, and lead-acid batteries that are currently commercially available as secondary batteries have the characteristics that they can be rapidly charged, but their battery voltage and energy density are low, and the requirements currently demanded. Can't answer. On the other hand, a so-called lithium ion battery using an intercalation compound of carbon doped with lithium ions or a graphite intercalation compound as a negative electrode material is a nickel-cadmium battery,
It has higher voltage and higher energy density than nickel-metal hydride batteries and lead storage batteries. However, this lithium ion battery has the following four problems to be solved. (Problem 1) It is inferior in both voltage and energy density as compared with a lithium secondary battery using metallic lithium for the negative electrode. (Problem 2) Since diffusion of lithium ions in the negative electrode is slow, high-current discharge rapid charging cannot be performed. (Problem 3) If only the negative electrode is changed to metallic lithium in order to increase the voltage and energy density, the charge / discharge efficiency of the negative electrode is particularly low, which shortens the charge / discharge cycle life and requires optimization of each material. Furthermore, (Problem 4)
The manufacture of lithium-ion batteries involves a special process, which makes the cost of manufacturing large-capacity batteries very high. A non-aqueous solvent is used as a solvent for an electrolytic solution in a lithium battery. As the non-aqueous solvent, a cyclic ester such as propylene carbonate (PC) is used because it has a higher dielectric constant and is relatively stable than before. . However, when this propylene carbonate is used as a single-solvent electrolyte, the conductivity is relatively low, so the negative electrode characteristics and low temperature characteristics are remarkably inferior, and the lithium charging / discharging efficiency is also low. Therefore, propylene carbonate is rarely used as a single-solvent electrolytic solution, and is used as an electrolytic solution mixed with a low-viscosity solvent such as 1,2-dimethoxyethane (DME).

[0003]

Since the electrolyte using 1,2-dimethoxyethane is not high in the oxidation resistance of 1,2-dimethoxyethane, the capacity of the lithium battery deteriorates when stored in a charged state. There was a problem that cycle deterioration under high temperature and temperature became large. For such a problem,
It has been proposed to use a mixture of ethylene carbonate and dimethyl carbonate as the electrolyte. However, this mixed solvent electrolyte has a problem that the melting point of the solvent is relatively high at 36 ° C. for ethylene carbonate and 3 to 4 ° C. for dimethyl carbonate, so that the capacity cannot be sufficiently improved at low temperature mainly due to solidification. The present invention is intended to solve such a problem of the conventional technology,
We have found a non-aqueous solvent system that replaces the conventional mixed solvent, and use a non-aqueous solvent electrolyte that has high conductivity and good temperature characteristics and negative electrode characteristics, and also has excellent redox resistance, and is suitable for the positive and negative electrodes. An object of the present invention is to provide a lithium secondary battery having a combination of high energy density, high-current discharge rapid charge, long charge / discharge life, and low cost.

[0004]

Briefly, the present invention relates to a secondary battery, which is a negative electrode capable of charging and discharging lithium ions and a positive electrode capable of reversible electrochemical reaction with lithium ions. , And a secondary battery having an electrolytic solution in which an ion dissociative lithium salt is dissolved in a non-aqueous solvent,
As a non-aqueous solvent of the electrolytic solution, dimethyl carbonate,
It is characterized by using a mixed solvent containing ethylene carbonate and methyl acetate.

As a result of various studies to achieve the above-mentioned object, the inventors of the present invention have selected three types of non-aqueous solvents, ethylene carbonate, dimethyl carbonate and methyl acetate, as the non-aqueous solvent used in the non-aqueous solvent electrolyte. It was found that it is effective to use a mixed solvent containing a solvent, and the present invention has been completed. That is, the present invention uses a positive electrode active material that requires a cut-off voltage of 3.5 V or higher for the positive electrode, and can charge and discharge lithium ions in the negative electrode material, especially non-aqueous materials using metallic lithium or a lithium metal alloy. In a secondary battery having a solvent electrolyte, a non-aqueous solvent electrolyte that enables large-current discharge and rapid charging by using a mixed solvent containing ethylene carbonate, dimethyl carbonate, and methyl acetate as a solvent of the non-aqueous solvent electrolyte. It is possible to provide a secondary battery having

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. The present invention is characterized by containing three kinds of ethylene carbonate, dimethyl carbonate, and methyl acetate as a non-aqueous solvent of the electrolytic solution. Here, as a mixing ratio of each solvent constituting the non-aqueous solvent, 10 to 60% by volume of ethylene carbonate and 2 of dimethyl carbonate
It is preferable that 0 to 60% by volume and methyl acetate be 5 to 40% by volume. Further, the volume mixing ratio of ethylene carbonate and the total amount of dimethyl carbonate and methyl acetate is preferably 10:90 to 50:50. In addition, since all the substituents are methyl groups even in transesterification between chain esters, which is caused by the charge / discharge cycle of the battery, it is possible to prevent the generation of a solvent such as diethyl carbonate which lowers the charge / discharge efficiency. If the ethylene carbonate content is less than 10% by volume, the cycle life of the secondary battery may be impaired, while if it exceeds 60% by volume, solidification may occur at room temperature, and the dimethyl carbonate, methyl acetate, If the amount of each decreases, as a result, large current discharge, rapid charging characteristics, and low temperature characteristics may not be improved satisfactorily, while if the former exceeds 60% by volume and the latter exceeds 40% by volume, cycle life may deteriorate. There is. Further, the volume mixing ratio of ethylene carbonate and the total amount of dimethyl carbonate and methyl acetate is preferably 10:90 to 50:50,
If the ethylene carbonate content is less than 10% by volume, the cycle life may be deteriorated, while if it exceeds 50% by volume, the high load, large current discharge, rapid charging characteristics and low temperature characteristics may not be improved satisfactorily.

Further, the electrolyte of the electrolytic solution is not particularly limited, and examples thereof include LiClO ₄ , LiPF ₆ and LiAsF.
₆ , LiBF ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ ,
LiSbF ₆ , LiSCN, LiCl, LiC ₆ H ₅ S
O ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO
₂ ) ₃ and lithium salts such as LiCF ₃ SO ₃ can be used alone or in combination of two or more. Of these, especially
It is preferable to use LiPF ₆ , LiAsF ₆ , or LiClO ₄ . As the positive electrode, in the present invention, a positive electrode active material which requires a charge end voltage of 3.5 V or more, particularly Li _x Mn _{2- y My} O in order to enable high energy density.
₄ (M = Na, Mg, Sc, Y, Fe, Co, Ni, C
u, Zn, Al, Pb, Sb, 0 ≦ x ≦ 1.2, 0 <y
≦ 0.7) as a main constituent oxide or Mn ₂ O ₄
It is preferable to use a complex oxide mainly containing. As a negative electrode material capable of charging and discharging lithium ions, 1) a lithium metal negative electrode, 2) a lithium alloy negative electrode capable of charging and discharging lithium ions, for example, a lithium alloy mainly containing Li and Al, Li and Cd, In, Lithium alloys with Pb, Bi, etc., 3) Negative electrodes mainly composed of a negative electrode active material holder capable of charging and discharging lithium ions, such as various carbon materials, Nb ₂ O ₅ , WO ₂ , Fe ₂ O _3, etc. Metal oxides,
Polymer compounds such as polythiophene and polyacetylene, L
i _2.5 Co _0.5 N, Li _2.5 Cu _0.5 N, Li _2.5 Ni
Nitride such as _0.5 N, Li ₃ FeN ₂ , and Li ₇ MnN ₄ can be used.

The secondary battery having the non-aqueous solvent electrolyte of the present invention has the following features. That is, as the positive electrode active material, Li _x Mn _{2- y My} O ₄ (M = Na, M
g, Sc, Y, Fe, Co, Ni, Cu, Zn, Al,
Pb, Sb, 0 ≦ x ≦ 1.2, 0 <y ≦ 0.7) and M
A battery using a complex oxide mainly composed of n ₂ O ₄ is characterized by being inexpensive and having a long cycle life. Further, as a positive electrode active material, Li _x Mn _{2- y My} O ₄ (M = Na, M
g, Sc, Y, Fe, Co, Ni, Cu, Zn, Al,
A battery using a composite oxide mainly composed of Pb, Sb, 0 ≦ x ≦ 1.2, 0 <y ≦ 0.7) is Li _x Mn ₂ O.
By partially replacing ₄ (0 ≦ x ≦ 1.2) Mn with a transition metal, the crystal structure is particularly stable and the charge / discharge life is extended. Further, as a positive electrode active material, Li _x CoO ₂ (0 ≦ x
A battery using a complex oxide mainly composed of ≦ 1.2) has characteristics of high voltage and high energy density. Further, as a positive electrode active material, Li _x NiO ₂ (0 ≦ x ≦
The battery using the complex oxide mainly composed of 1.2) has the characteristics of large charge / discharge capacity and large energy density. Further, as a positive electrode active material, Fe ₂ (SO ₄ ) ₃
A battery using a complex sulfate mainly composed of is characterized by being inexpensive and lightweight. As described above, a high voltage and a high energy density can be obtained by using a positive electrode active material that requires 3.5 V or more as the final charge voltage for the positive electrode, and the lithium ion intercalation, It is suitable for high-current discharge rapid charging due to the rapid diffusion of deintercalation.

The negative electrode can have a high energy density by using metallic lithium or a lithium metal alloy, and since lithium ion intercalation or deintercalation is not necessary, the diffusion of lithium ion can be prevented. It does not cause any problems, and is therefore suitable for high-current discharge rapid charging, and it does not require a special step for producing the negative electrode, so that the cost can be reduced. The electrolyte is
Since the mixed solvent consisting of ethylene carbonate and dimethyl carbonate, which has been proposed as a non-aqueous solvent for improving the cycle characteristics, is mixed with methyl acetate, which is a low-viscosity and low-melting-point solvent, the electrolytic solution is It is possible to have both high conductivity and low freezing point at the same time.
Therefore, it becomes possible to sufficiently improve the load characteristics and low temperature characteristics of the battery.

[0010]

EXAMPLES The effects of the present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples. In each of the following examples, ethylene carbonate is EC, dimethyl carbonate is DMC, and methyl acetate is M.
Abbreviated as A.

Examples 1 to 7 and Comparative Example 1 FIG. 1 is a sectional view of a secondary battery having a non-aqueous solvent electrolyte according to the present invention. In FIG. 1, 1 is a negative electrode case made of stainless steel. Reference numeral 2 denotes a negative electrode, which is obtained by punching out a lithium foil having a predetermined thickness to a diameter of 16 mm and press-bonding it to 1. Reference numeral 3 is an electrolytic solution using a non-aqueous solvent, which is prepared by dissolving 1 mol / liter of lithium hexafluorophosphate LiPF ₆ in a mixed solvent having various compositions of EC, DMC and MA. 4 is a separator made of a polypropylene or polyethylene porous film. 5
Is a stainless steel positive electrode case. 6 is LiMn _1.9 C
It is a positive electrode composed of _0.1 O ₄ . This is prepared by mixing the above positive electrode active material with a conductive agent and a binder to form a slurry, applying the slurry to a predetermined thickness on a SUS foil, and drying it.
It was cut into the size of. Reference numeral 7 denotes a gasket, and the negative electrode case 1 maintains electrical insulation between the negative electrode case 1 and the positive electrode case 5, and at the same time, the opening edge of the negative electrode case is bent inward.
By caulking, the battery contents are hermetically sealed.

With respect to the coin type batteries of Examples 1 to 7 and Comparative Example 1 produced as described above, the charge end voltage is 4.3 V and the discharge end voltage is 3.3 V at 20 ° C. in order to evaluate the high load characteristics. For 10 cycles, the current density was 1 mA / cm ² and the discharge current was 1 mA / cm ² . After that, after charging under the same conditions, a relatively high load of 6 mA / cm
^The discharge was performed at ² and the discharge capacity at that time was estimated. In addition to this, a charging / discharging cycle was performed as in the case of evaluating the high load characteristic in order to evaluate the low temperature characteristic, and thereafter, the temperature was set to −10 ° C. and then discharged at 6 mA / cm ² ,
The discharge capacity at that time was estimated. Table 1 shows the above results. The high-load characteristic discharge capacity ratio and the low-temperature characteristic discharge capacity ratio are shown as values with the discharge capacity of Comparative Example 1 as 100. From the results in Table 1, it is clear that the battery of the present invention using the electrolytic solution containing three kinds of EC, DMC and MA is excellent not only in normal cycle characteristics but also in high load characteristics and low temperature characteristics. .

[0013]

[Table 1]

[0014]

As described above, according to the present invention, the conductivity of the electrolytic solution in the secondary battery having the non-aqueous solvent electrolytic solution becomes very high, and the temperature characteristic and the load characteristic are excellent.
Further, it also has excellent redox resistance. This allows
It is possible to provide a secondary battery having a high energy density, capable of rapid charging with a large current discharge, a long charge / discharge life, and an inexpensive nonaqueous solvent electrolyte.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a battery of the present invention.

[Explanation of symbols]

1: Stainless steel negative electrode case, 2: Negative electrode, 3: Electrolyte using non-aqueous solvent, 4: Separator, 5: Stainless steel positive electrode case, 6: Positive electrode, 7: Gasket

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H01M 10/40 H01M 10/40 Z

Claims (8)

[Claims] 1. A negative electrode capable of charging and discharging lithium ions,
In a secondary battery having a positive electrode capable of reversibly electrochemically reacting with lithium ions, and an electrolytic solution in which an ion dissociative lithium salt is dissolved in a non-aqueous solvent, dimethyl carbonate and ethylene carbonate are used as the non-aqueous solvent of the electrolytic solution. And a mixed solvent containing methyl acetate is used. 2. A non-aqueous solvent containing 1 part of ethylene carbonate.
The secondary battery having a non-aqueous solvent electrolytic solution according to claim 1, wherein the secondary battery contains 0 to 60% by volume, 20 to 60% by volume of dimethyl carbonate, and 5 to 40% by volume of methyl acetate. 3. The volume mixing ratio of ethylene carbonate to the total amount of dimethyl carbonate and methyl acetate is 1.
The secondary battery having the non-aqueous solvent electrolyte according to claim 2, which is 0:90 to 50:50. 4. The secondary battery having a non-aqueous solvent electrolyte according to claim 3, wherein metallic lithium or a lithium metal alloy is used as a negative electrode active material for the negative electrode. 5. The nonaqueous solvent electrolyte solution according to claim 1, wherein the positive electrode is one that requires a cutoff voltage of charge of 3.5 V or more. Secondary battery. 6. Li _x Mn _2-y M as the positive electrode active material
_y O ₄ (M = Na, Mg, Sc, Y, Fe, Co, N
i, Cu, Zn, Al, Pb, Sb, 0 ≦ x ≦ 1.2,
The secondary battery having a non-aqueous solvent electrolyte according to claim 5, characterized in that 0 <y ≦ 0.7) or a composite oxide mainly composed of Mn ₂ O ₄ is used. 7. A positive electrode active material comprising Li _x Mn _2-y Co _y
The secondary battery having a non-aqueous solvent electrolyte according to claim 6, wherein O ₄ (0 ≦ x ≦ 1.2, 0 <y ≦ 0.7) is used. 8. LiP as a lithium salt for the electrolytic solution
0.5% of F ₆ , LiAsF ₆ or LiClO ₄
The secondary battery having the non-aqueous solvent electrolyte according to any one of claims 1 to 7, which is used at a concentration of 1.5 mol / liter.