JPH04206167A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To obtain a secondary battery having an excellent charge/discharge cycle life and a high capacity by using a material obtained by mixing a prescribed mix volume of acetonitrile with one kind among ethylene carbonate, propylene carbonate or butylene carbonate as nonaqueous solvent. CONSTITUTION:A carbonaceous material of a negative electrode 3 in a nonaqueous electrolyte secondary battery is made to bear Li thereon, and a chalcogen compound such as Mn oxide is contained in a positive electrode 1 as an active material, and hexafluoride lithium phosphate is used as nonaqueous electrolyte after about 0.5-1.5 mole/l of it is dissolved into nonaqueous solution as electrolyte. The nonaqeous solution is formed by mixing acetonitrile of about 10-60volume% with one kind among ethylene carbonate, propylene carbonate and butylene carbonate. At this time, diameter of an Li ion solvation complex in the nonaqueous electrolyte can be reduced. Thereby, the complex can infiltrate easily between layers of a carbonic material of the negative electrode 3 so that storage/emission volume of Li ion can be increased at the negative electrode. Moreover, since there is no reactivity between the nonaqueous solution and the carbonic material, a structure of the carbonic material can be kept in an excellent condition so that the secondary battery having high capacity can be made.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention (Industrial Application Field) The present invention relates to a nonaqueous electrolyte secondary battery, and particularly to a nonaqueous electrolyte secondary battery with an improved nonaqueous electrolyte.

(Prior Art) In recent years, with the miniaturization and weight reduction of various electronic devices, the demand for high energy density secondary batteries as their power sources has increased. For this reason, it is attracting attention as a nonaqueous electrolyte secondary battery or a high energy density secondary battery that uses a light metal such as lithium as a negative electrode active material.

The non-aqueous electrolyte secondary battery uses lithium for the negative electrode and propylene carbonate (PC) as the electrolyte.
, 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-BL) in a non-aqueous solvent such as tetrahydrofuran (THF), LiCjl) 04, LiB F 4
, L I As F 6 , L I P F b, etc. are used, and TI is mainly used as the positive electrode active material.
Various studies have been conducted on methods using chalcogen compounds that undergo topochemical reactions with lithium, such as S2, MOS2, V20s, and V6013.

However, the above-mentioned non-aqueous electrolyte secondary battery has not yet been put into practical use. The main reason for this is that the charging/discharging efficiency is low and the number of charging/discharging cycles (cycles) life is short. This is thought to be largely due to deterioration of lithium due to the reaction between the negative electrode lithium and the electrolyte. In other words, lithium dissolved in the electrolyte as lithium ions during discharging reacts with the solvent when precipitated during charging,
Part of its surface is inactivated. Therefore, when charging and discharging are repeated, phenomena such as dendrite-like (dendritic) lithium are generated, lithium is precipitated into small spheres, and lithium is detached from the current collector.

The above-mentioned deterioration of lithium is also greatly affected by the combination of electrolyte and nonaqueous solvent, so the optimal combination is being studied. For example, among various mixed solvents being considered as non-aqueous solvents, a mixed solvent of ethylene carbonate and 2-methyltetrahydrofuran is known to be a stable solvent for lithium. In the case of such a mixed solvent, L i A S F as the electrolyte
It has been reported that high lithium charge/discharge efficiency can be obtained with a non-aqueous electrolyte in which 1.5 mol/g of lithium b is dissolved (El
ectr.

-chei, Acta, 30.1715 (1985))
However, L,1AsF, has problems in terms of toxicity. For this reason, lithium hexafluorophosphate (L iP F 6) has a molar conductivity comparable to that of LiAsFb.
The use of lithium borofluoride (L iB F 4 ) is being considered. However, L I P F 6
and L I B F < have problems such as poor chemical stability, so it is difficult to obtain sufficient lithium charge/discharge efficiency with non-aqueous electrolytes in which these electrolytes are dissolved, and the storage characteristics are also poor. There is.

For this reason, a non-aqueous electrolyte secondary battery has been proposed in which a carbonaceous material capable of intercalating and deintercalating lithium ions carries lithium thereon as a negative electrode.

According to such a secondary battery, it is possible to improve the reactivity between lithium and the electrolytic solution, and to eliminate negative electrode deterioration due to dendrite precipitation and the like.

Even if a negative electrode in which lithium is supported on the carbonaceous material is used, the amount of intercalation and desorption of lithium ions is small, so the negative electrode specific capacity (mAh/g) is small.

For this reason, it has been difficult to simultaneously obtain sufficient battery capacity and charge/discharge cycle life.

(Invention or Problem to be Solved) The present invention was made to solve the problems of the conventional art, and aims to provide a non-aqueous solvent secondary battery with excellent charge/discharge cycle life and high capacity. be.

[Configuration of the Invention (Means for Solving the Problems) The present invention comprises a negative electrode in which lithium is supported on a carbonaceous material, a positive electrode in which a chalcogen compound is used as an active material, and lithium hexafluorophosphate (L i P F b) A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte dissolved in a non-aqueous solvent as an electrolyte, wherein the non-aqueous solvent contains ethylene carbonate, at least one of propylene carbonate and butylene carbonate, and acetonitrile. This is a non-aqueous electrolyte secondary battery comprising a mixture of acetonitrile and acetonitrile in an amount of 10 to 60% by volume.

The negative electrode can be manufactured, for example, as follows.

First, a carbonaceous material is obtained by carbonizing one or more organic compounds by firing and thermally decomposing them at 1000 to 3000'C for 12 to 3 hours in a non-oxidizing atmosphere.

In the cono step, a carbonaceous material may be obtained using coke or the like in addition to the organic compound described above.

Next, the carbonaceous material has a predetermined particle size (for example, an average particle size of 2
0 to 50 μm) to form a powder, and the powder and the binder are mixed in a predetermined ratio (e.g., 80 to 90:20 to 1 by weight).
0). By compressing and molding this mixed material into pellets or sheets, the carbonaceous material is made into a relatively porous carrier. Subsequently, the carbonaceous material is immersed in an electrolytic solution containing lithium ions at a predetermined concentration as an anode and metal lithium is used as a cathode for electrolytic treatment. Through this electrolytic treatment, a negative electrode in which lithium is supported on the carbonaceous material is obtained. In addition, chemical, physical, and
Lithium may be supported on the carbonaceous material by an electrochemical method. Further, if necessary, the carbonaceous material may further support an alkali metal such as sodium or aluminum.

Note that the active material such as lithium may be supported not only on the carbonaceous material but also on the positive electrode.

Examples of the organic compounds that are raw materials for the carbonaceous materials mentioned above include epoxy resins; phenol resins: acrylic resins such as polyacrylonitrile and poly(α-halogenated acrylonitrile); polyvinylidene chloride, polychlorinated vinyl chloride, etc. % rogenated bivinyl resin; Conjugated resin such as polyamide resin lyacetylene, poly(p-phenylene); Monocyclic carbonization such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, chrysene, naphthalene, picene, perylene, pentaphene, pentacene, etc. Various types of condensed polycyclic hydrocarbon compounds formed by condensing two or more hydrogen compounds, derivatives of these compounds such as carboxylic acids, carboxylic acid anhydrides, and carboxylic acid imides, and mixtures of these compounds as main components. At least two or more heterocyclic compounds such as pitch, indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole, acridine, phenazine, zenatridine, etc. are bonded to each other, or with one or more monocyclic hydrocarbon compounds. Fused heterocyclic compounds formed by bonding one or more heterocyclic compounds, derivatives of these compounds such as carboxylic acids, carposiic anhydrides, carposiimides, 2 Hensen's 1.2.4°5-tetracarboxylic acids, Examples include its dianhydride or its diimide.

Examples of the chalcochemistry compound used as the active material of the positive electrode include manganese oxides such as manganese dioxide and lithium manganese composite oxide, amorphous vanadium pentoxide, titanium dioxide, molybdenum disulfide, molybdenum selenide, and lithium cobalt composite oxide. and lithium cobalt nickel composite oxide.

The amount of lithium hexafluorophosphate dissolved in the nonaqueous electrolyte is preferably 0.5 to 1.5 mol/jll.

The reason for this is that if it is out of the above range, there is a risk of a decrease in electrical conductivity and a decrease in lithium charging/discharging efficiency.

The reason for limiting the amount of acetonitrile in the non-aqueous solvent of the non-aqueous electrolyte is that if the amount is 10% by volume, the mobility of lithium ions in the electrolyte will decrease. Exceeding this volume % will lead to deterioration of the negative electrode, etc. Furthermore, it is more preferable that the proportion of acetonitrile is 20 to 50% by volume.

The amount of ethylene carbonate and the amount of propylene carbonate or butylene carbonate in the nonaqueous solvent of the nonaqueous electrolyte are each 20 to 70%.
It is preferable to use volume %.

The non-aqueous electrolyte having the above-mentioned composition may be treated by contacting it with an insoluble adsorbent or energized before being placed in the battery container in order to remove impurities such as moisture and achieve high purity. , or both of these treatments are desirable.

As for the treatment with the insoluble adsorbent, for example, an insoluble adsorbent that does not react with the electrolytic solution, such as activated alumina or an inorganic molecular sieve, is added to the non-aqueous electrolytic solution and stirred, and then the insoluble adsorbent is removed by filtration or the like. A method of separation or a method of flowing the non-aqueous electrolyte through a column filled with the insoluble adsorbent can be adopted.

The energization treatment may be performed, for example, by immersing an electrode made of lithium as an anode in the non-aqueous electrolyte and immersing an electrode made of lithium or a metal other than lithium as a cathode, and then applying a constant current between the anode and the cathode. Alternatively, a method may be adopted in which lithium is deposited on the cathode by applying continuous waves or pulses at a constant voltage, or the deposition and dissolution are repeated.

(Function) According to the present invention, a negative electrode in which lithium is supported on a carbonaceous material, a positive electrode in which a chalcochemistry compound is used as an active material, and a non-aqueous electrolyte in which lithium hexafluorophosphate is dissolved in a non-aqueous solvent as an electrolyte. In the nonaqueous electrolyte secondary battery, the nonaqueous solvent is made of a mixture of ethylene carbonate, at least one of propylene carbonate and butylene carbonate, and acetonitrile, and the amount of acetonitrile is 10 to 60%. By setting it as volume %, the diameter of the solvated complex of lithium ions in the non-aqueous electrolyte can be reduced. Therefore, the complex easily penetrates into the interlayers of the carbonaceous material of the negative electrode, and the amount of lithium ions absorbed and released in the negative electrode can be increased. Furthermore, since there is no reactivity between the non-aqueous solvent and the carbonaceous material, the structure of the carbonaceous material can be maintained well. As a result, high capacity can be achieved.

In addition, the conditions for the non-aqueous solvent in the non-aqueous electrolyte to obtain a secondary battery with a long charge-discharge cycle life are: ■ Low electron transfer reactivity with lithium; ■ Electrolyte consisting of lithium salt (L i P (2) low viscosity that affects the mobility of lithium ions; and (2) ability to stably dissolve the electrolyte (LiPF'6). The ethylene carbonate, propylene carbonate, and butylene carbonate have the advantage of having low reactivity with lithium, high dielectric constant, high ionic dissociation, and being able to stably dissolve the electrolyte, but have the disadvantage of high viscosity. be. For this reason, by blending low-viscosity acetonitrile with the ethylene carbonate and at least one of propylene carbonate and butylene carbonate at a ratio of 10 to 60% by volume, the advantages of the ethylene carbonate, etc. are not lost. The viscosity can be lowered to fully satisfy the conditions (1) to (3) above, and the charge/discharge cycle life can be improved.

Furthermore, the acetonitrile has a high oxidation potential and is not decomposed even when charged to 4.8V. In addition, lithium hexafluorophosphate (L iP F 6 ) as an electrolyte is not decomposed even when charged to a high oxidation potential of <4.8V. Therefore, even if the battery is repeatedly charged and discharged at a battery voltage of 3.0 to 4.5V, the non-aqueous electrolyte does not change in quality and has excellent stability.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Example 1 First, 60 g of manganese dioxide and 12.7 g of lithium carbonate
were thoroughly mixed and ground in a mortar (molar ratio of Mn:
Li-2:1). Next, this was heated in air at 850°C for 5 hours, cooled, and then heated again in air at 850°C.
Heat treatment was performed for 3 hours.

When the obtained product was examined by X-ray diffraction, Li
It was confirmed that a Mn2O4 phase was generated. Subsequently, 90 parts by weight of the obtained product (positive electrode active material), 10 parts by weight of acetylene black as a conductive material, and 5 parts by weight of polytetrafluoroethylene as a binder were kneaded to prepare a positive electrode mixture. did. Press this mixture to about 2 tons/
It was press-molded under conditions of cm 2 and further dried in a vacuum at 200° C. to produce a positive electrode with a diameter of 15.7 mm.

On the other hand, phenol resin powder was heated to 17% in nitrogen gas.
It was baked at 00°C for 2 hours. The obtained carbonaceous material is crushed,
95 parts by weight of this powder and 5 parts by weight of polyethylene powder as a binder were kneaded, and 200 mg of this kneaded product was pressure-molded to form a pellet-shaped carrier with a thickness of 0.8 mm.

Subsequently, the support was immersed in an electrolytic solution containing lithium ions, with the support as an anode and lithium as a cathode, at an electrolytic solution temperature of 20° C. and a current density of 0.5 m A/c rn 2.
An electrolytic treatment was performed under the following conditions. Through this electrolytic treatment, a negative electrode in which 30 mAh of lithium was supported on the support was produced.

In addition, a mixture of ethylene carbonate, propylene carbonate, and acetonitrile (volume ratio, ethylene carbonate:propylene carbonate:acetonitrile-3
Lithium hexafluorophosphate (L i P F 6 ) was dissolved in a non-aqueous solvent consisting of 0:30:40) at a concentration of 1 mol/g. Next, an anode and a cathode each made of a lithium plate were placed in this solution, and the current density was 1 mA.
A non-aqueous electrolyte was prepared by applying a current of /cm2 for 10 hours or more.

Next, a button-shaped nonaqueous electrolyte secondary battery as shown in FIG. 1 was assembled using the positive electrode, negative electrode, and nonaqueous electrolyte. That is, a separator 2 made of a polypropylene nonwoven fabric impregnated with the non-aqueous electrolyte is placed on the positive electrode 1 . A negative electrode 3 is placed on the separator 2. The positive electrode 1, separator 2, and negative electrode 3 are housed in a battery container composed of a positive electrode can 4 and a negative electrode can 5 made of stainless steel, so that the positive electrode 1 and the positive electrode can 4 face each other. An insulating backing 6 is interposed between the positive electrode can 4 and the negative electrode can 5. A current collector 7 crimped onto the bottom surface of the positive electrode 1 is interposed between the positive electrode 1 and the positive electrode can 4 .

Example 2 A mixture of ethylene carbonate, butylene caponate, and acetonitrile (volume ratio, ethylene carbonate:butylene carbonate:acetonitrile-30:30:
A non-aqueous electrolyte secondary battery was assembled in the same manner as in Example 1, except that 40) was used as the non-aqueous solvent of the non-aqueous electrolyte.

Comparative Example 1 A non-aqueous electrolyte was prepared in the same manner as in Example 1, except that a mixture of ethylene carbonate and propylene caponate (volume ratio, ethylene carbonate: propylene carbonate - 50:50) was used as the non-aqueous solvent of the non-aqueous electrolyte. An electrolyte secondary battery was assembled.

Comparative Example 2 Non-aqueous electrolysis was performed in the same manner as in Example 1, except that a mixture of ethylene carbonate and butylene caponate (volume ratio: ethylene carbonate, butylene caponate -50+50) was used as the non-aqueous solvent of the non-aqueous electrolyte. I assembled a liquid secondary battery.

Comparative Example 3 A non-aqueous electrolyte secondary battery was assembled in the same manner as in Example 1, except that a negative electrode made of lithium foil was used.

Regarding the non-aqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3, the battery voltage was set to 3 OV at a current IIIA/CI2.
Charging from 4.5V to 4.5V and increasing the battery voltage from 4.5V to 3.0V with a current of 1rAA/co+2. Discharging to OV was repeated, and the discharge capacity at each number of charge/discharge cycles was measured. The results are shown in FIG. Furthermore, the discharge capacity retention rate at each number of charge/discharge cycles was determined from the discharge capacity. The results are shown in FIG.

As is clear from FIGS. 2 and 3, the battery of Example 1-2 has a larger discharge capacity and superior discharge capacity retention rate than the batteries of Comparative Examples 1 and 2. Further, although the battery of Comparative Example 3 has a large initial discharge capacity, the negative electrode deteriorates, so the discharge capacity retention rate decreases significantly.

It should be noted that the batteries of Examples 1 and 2 can both be button-shaped, or the same effect can be obtained even if the batteries are cylindrical, flat, prismatic, or the like.

[Effects of the Invention] As detailed above, according to the present invention, it is possible to provide a non-aqueous solvent secondary battery with excellent charge/discharge cycle life and high capacity.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing the non-aqueous solvent secondary battery of Example 1, and Figure 2 shows the discharge capacity versus the number of charge/discharge cycles in the non-aqueous solvent secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3. FIG. 3 is a characteristic diagram showing changes in the discharge capacity retention rate with respect to the number of charge/discharge cycles in the non-aqueous solvent secondary batteries of Example 1.2 and Comparative Examples 1 to 3. ■...Positive electrode, 2...Separator, 3...Negative electrode, 4
...Positive electrode can, 5...Negative electrode can. Applicant's agent Patent attorney Takehiko Suzue Jll

Claims (1)

[Claims] A nonaqueous electrolyte comprising a negative electrode in which lithium is supported on a carbonaceous material, a positive electrode in which a chalcogen compound is used as an active material, and a nonaqueous electrolyte in which lithium hexafluorophosphate (LiPF_6) is dissolved in a nonaqueous solvent as an electrolyte. In the liquid secondary battery, the non-aqueous solvent is made of a mixture of ethylene carbonate, at least one of propylene carbonate and butylene carbonate, and acetonitrile, and the amount of acetonitrile is 10 to 60% by volume. Characteristic non-aqueous electrolyte secondary battery.