JPH0652886A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Summary] [Object] To provide a non-aqueous electrolyte secondary battery having excellent cycle life characteristics and low temperature characteristics by improving the solvent of the electrolyte solution. [Structure] A negative electrode made of a carbon material capable of inserting and extracting lithium ions, a non-aqueous electrolyte solution, and a positive electrode made of a lithium-containing oxide are provided, and the solvent of the non-aqueous electrolyte solution is an ester, and methyl propionate is used. I decided to include it. As a result, a non-aqueous electrolyte secondary battery having excellent cycle life and low temperature characteristics can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improving the cycle life and capacity characteristics of this battery at low temperatures.

[0002]

2. Description of the Related Art In recent years, portable electronic devices and cordless electronic devices have been rapidly developed, and there is a great demand for a small and lightweight secondary battery having high energy density as a power source for driving these electronic devices. In this regard, non-aqueous electrolyte secondary batteries,
In particular, lithium secondary batteries are highly expected as batteries having high voltage and high energy density.

When a non-aqueous electrolyte battery is used as a secondary battery, a positive electrode active material having a high capacity and a high voltage is desired.
LiCoO ₂ and LiMn ₂ O satisfy the requirements.
A material showing a high voltage of 4V of ₄ series is mentioned.

On the other hand, as negative electrode materials, metallic lithium, lithium alloys, and carbon materials capable of absorbing and releasing lithium ions have been investigated. However, metallic lithium has a problem of short circuit due to dendritic products (dendrites) associated with charge and discharge, and lithium alloy has a problem of electrode collapse due to expansion and contraction associated with charge and discharge. Therefore, recently, carbon materials that do not cause these problems have been regarded as promising as negative electrode materials for lithium secondary batteries.

In general, when metallic lithium is used as a negative electrode material, active dendrites formed on the negative electrode surface during charging react with a non-aqueous solvent to cause a partial solvent decomposition reaction, which lowers charging efficiency. It is well known. As a means for solving this, JP-A-57-17046
In Japanese Patent Publication No. 3, attention is paid to the fact that ethylene carbonate is excellent in charging efficiency, and it is proposed to use a mixed solvent of ethylene carbonate and propylene carbonate. Further, in JP-A-3-55770, in order to improve the low temperature characteristics of the battery, 2-methyltetrahydrofuran, 1,2-dimethoxyethane and 4-methyl-1,3- are used in a mixed solvent of ethylene carbonate and diethyl carbonate.
It has been proposed to mix dioxolane or the like and use it as a solvent for a non-aqueous electrolyte.

However, even if these systems are used, the charging efficiency is limited to about 98 to 99% at the maximum, and the charging efficiency is not yet sufficiently increased. This is the same even when a lithium alloy is used for the negative electrode.

[0007]

When a carbon material is used as the negative electrode material, the charging reaction is a reaction in which lithium ions in the electrolytic solution intercalate between the layers of the carbon material, so that dendrite of lithium is produced. Therefore, the decomposition reaction of the solvent on the surface of the negative electrode should not occur.
However, in reality, the charging efficiency is less than 100%, and the same problems as in the case of using lithium or a lithium alloy for the negative electrode remain.

The present inventors have not found that this phenomenon is due to the decomposition reaction of the solvent on the surface of the negative electrode as in the case where lithium metal is used for the negative electrode, but when lithium intercalates between the layers of the negative electrode carbon material, It was considered that not only lithium but also a solvent coordinated with lithium was drawn in between the layers, and at that time, a decomposition reaction of a part of the solvent was caused. That is, the solvent having a large molecular radius is not smoothly intercalated between the layers of the negative electrode carbon material, but is decomposed at the inlet between the layers of the negative electrode material.

Generally, a requirement for an excellent solvent for an electrolytic solution of a lithium battery is that it has a large dielectric constant, that is, it can dissolve a large amount of an inorganic salt as a solute. Cyclic esters such as propylene carbonate and ethylene carbonate are said to be excellent solvents that satisfy this requirement, but since these all have a large molecular radius due to their cyclic structure, when a carbon material is used for the negative electrode, As described above, there is a problem in that the solvent is decomposed during charging. Further, since these solvents are highly viscous, when used alone, they have a drawback that the viscosity of the electrolytic solution is high and high rate charge / discharge is difficult, and the capacity at low temperature is small. In particular, ethylene carbonate has a high freezing point of 36.4 ° C. and cannot be used alone. As mentioned above, as a method of compensating for this drawback, a method of mixing ethers such as 2 methyltetrahydrofuran, 1,2 dimethoxyethane, and 4 methyl 1,3 dioxolane is generally taken, but on the other hand, these ethers are mixed. Has a low oxidative decomposition voltage, and when used in a secondary battery, a decomposition reaction occurs during charging, resulting in a short cycle life.

On the other hand, due to the structure of the chain ester, the chain ester easily enters between the layers of the carbon material, and the decomposition reaction at the time of charging hardly occurs. Among them, it is already known in U.S. Pat. No. 4,804,596 that methyl acetate and methyl formate are excellent solvents, but on the contrary, these solvents have relatively high reactivity with lithium and are used as negative electrode materials. Even when a carbon material is used, it has a drawback that it partially reacts with lithium when entering the layer and is gradually consumed when the charge / discharge cycle is repeated. In addition, many of them have relatively low boiling points, which poses a problem that they are difficult to handle when constructing a battery.

The present invention has been made to solve the above problems, and its main object is to provide a non-aqueous electrolyte secondary battery having a long life and an excellent capacity retention at low temperatures. Is.

[0012]

In order to solve the above-mentioned problems and achieve the above-mentioned object, the present invention uses a solvent composed of an ester containing methyl propionate as a solvent of an electrolytic solution.

[0013]

The present inventors have found that the chain ester of methyl acetate, methyl formate and the like has a relatively high reactivity with lithium.
This is because the functional group is a carboxyl group or a methylcarboxyl group and the chain structure is relatively short.Esters with a longer chain structure are stable to lithium, but on the other hand, if the chain is too long, on the contrary, As described above, it has been found that a solvent is apt to undergo a decomposition reaction when it is drawn between layers of a carbon material, and in that sense, methyl propionate having a functional group of ethylcarboxyl is an excellent solvent.

At the same time, since this methyl propionate has a low freezing point, it is not necessary to mix and use ethers having a low oxidative decomposition voltage, and it exhibits excellent low-temperature characteristics.
It has been found that it has a higher boiling point than methyl acetate and methyl formate and is therefore easy to handle.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. In the examples, a cylindrical battery was constructed and evaluated.

(Embodiment 1) FIG. 1 shows a vertical sectional view of a cylindrical battery. In the figure, 1 indicates a positive electrode, which is an active material Li.
CoO ₂ mixed with carbon black as a conductive material and an aqueous dispersion of polytetrafluoroethylene as a binder in a weight ratio of 100: 3: 10 was applied to both sides of an aluminum foil and dried, After being rolled, it is cut into a predetermined size. To this, 2 titanium lead plates are spot welded. The mixing ratio of the aqueous dispersion of polytetrafluoroethylene as the binder is calculated by its solid content. Reference numeral 3 denotes a negative electrode, which is composed mainly of a carbonaceous material, and the weight ratio of the carbonaceous material and the acrylic binder is 100: 5.
The nickel foil is applied on both sides of the mixture, dried, rolled, and then cut into a predetermined size. The nickel negative electrode lead plate 4 is also spot-welded to this. Reference numeral 5 denotes a separator made of a polypropylene microporous film, which is interposed between the positive electrode 1 and the negative electrode 3 and is wholly wound in a spiral to form an electrode plate group. Insulating plates 6 and 7 made of polypropylene are arranged at the upper and lower ends of the electrode plate group, respectively, and inserted into a case 8 made of nickel plated with iron. Then, the positive electrode lead 2 is attached to the titanium sealing plate 10,
After spot welding the negative electrode lead 4 to the bottom of the case 8, a predetermined amount of electrolytic solution is injected into the case, and the battery is sealed with the sealing plate 10 through the gasket 9 to complete the battery. The size of this battery is 14 mm in diameter and 50 mm in height.
In addition, 11 is a positive electrode terminal of the battery, and the battery case 8 also serves as a negative electrode terminal.

The solvent of the electrolytic solution contains propylene carbonate (hereinafter referred to as PC) which is a cyclic ester, methyl formate (hereinafter referred to as MF) which is a chain ester, methyl acetate (hereinafter referred to as MA), methyl propionate (hereinafter referred to as MP). And a cyclic ester using ethylene carbonate (hereinafter referred to as EC), a chain ester diethyl carbonate (hereinafter referred to as DEC), and a chain ether dimethoxyethane (hereinafter referred to as DME). For the chain ester / chain ether mixed system (shown by volume ratio), the following trial production of cylindrical batteries A to E was performed. Lithium hexafluorophosphate was used as the solute of the electrolytic solution, and the concentration was adjusted to 1 mol / l.

Battery A ... PC = 100 Battery B ... MF = 100 Battery C ... MA = 100 Battery D ... MP = 100 Battery E ... EC: DEC: DME = 20: 40: 40 Evaluated battery characteristics Are cycle life characteristics and low temperature characteristics.

The test conditions were a charge / discharge current of 100 mA, a charge end voltage of 4.2 V, and a discharge end voltage of 3.0 V.
After charging / discharging the cycle at 20 ° C., the test was stopped in the charged state, the temperature was changed to −10 ° C. to discharge, and the low temperature characteristics were evaluated by the magnitude of the discharge capacity. Then the temperature is 20 ℃
Back to the same condition, repeated charging and discharging under the same voltage and current conditions, and when the discharge capacity deteriorates to 50% of the initial value, the test ends,
The number of cycles was defined as the cycle life.

The cycle life characteristics of the batteries A to E are shown in FIG. From FIG. 2, in order of good cycle life characteristics, D-C-B-
It became EA. Above all, A using a cyclic ester alone
Has a particularly short cycle life. At the time of charging, lithium ions intercalate between the layers of the carbon material at the negative electrode during charging, but at this time, solvent molecules coordinated to the lithium ions are also drawn in between the layers, so that a solvent having a cyclic structure and large molecules is used. It is thought that this is due to partial decomposition. Next, it is considered that the reason why the cycle life of E is short is that DME which is an ether solvent is decomposed during charging as described above. Also,
Cycle life characteristics are MP in the chain ester alone system
The result is that the order of -MA-MF is good. From this, it is considered that the longer the chain structure of the functional group of the chain ester is, the more stable it is, the reactivity with lithium is suppressed, and the good cycle characteristics are given.

From the above results, it was the single-solvent system of the chain ester that had good cycle life characteristics, and the chain ester was the longest in the case where the chain ester was the methyl propionate of the present invention.

Next, as shown in FIG. 3, B, C and D were almost unchanged in order of good low temperature characteristics, and then E and A.

From the above results, it was the methyl propionate alone system according to the present invention that both cycle life characteristics and low temperature characteristics were good.

Although the composite oxide of lithium and cobalt was used as the positive electrode active material in the examples, other composite oxides of lithium and nickel, composite oxides of lithium and manganese, and composite oxides of lithium and iron were used. Lithium-containing oxides such as, or cobalt of each of the above composite oxides,
Similar results were obtained with nickel, manganese, and iron partially substituted with other transition metals.

In the present embodiment, lithium hexafluorophosphate was used as the solute of the electrolytic solution, but other lithium-containing salts such as lithium borofluoride, lithium perchlorate, lithium trifluoromethanesulfonate, and hexafluorofluoride. Similar results were obtained with lithium oxide and the like.

In this embodiment, the low temperature characteristic test is -1.
Although it was carried out at 0 ° C., if a lower temperature is required, there is a method of mixing methyl propionate with a solvent having a lower freezing point. Similarly, when using at a higher temperature, there is also a method of mixing and using a high boiling point solvent. However, in those cases, the cycle life characteristics may deteriorate, but when importance is attached to the low temperature characteristics, those methods can be adopted.

[0027]

As is apparent from the above description, according to the present invention, by using the ester as the solvent of the electrolytic solution and containing methyl propionate in the ester, excellent cycle life characteristics and low temperature characteristics are obtained. A non-aqueous electrolyte secondary battery can be provided.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a cylindrical battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing the cycle life of the battery in Example at 20 ° C.

FIG. 3 is a diagram showing a transition of discharge voltage at −10 ° C. of a battery in an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Positive electrode lead plate 3 Negative electrode 4 Negative electrode lead plate 5 Separator 6 Upper insulating plate 7 Lower insulating plate 8 Case 9 Gasket 10 Sealing plate 11 Positive electrode terminal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takayuki Kawahara 1 Toho-cho, Yokkaichi-shi, Mie Prefecture Yokkaichi Research Institute Ltd. (72) Inventor Katsuaki Hasegawa 1 Toho-cho, Yokkaichi-shi, Mie Mitsubishi Petrochemical Incorporated company Yokkaichi Research Institute

Claims (3)

[Claims] 1. A negative electrode made of a carbon material capable of inserting and extracting lithium ions, a non-aqueous electrolytic solution, and a positive electrode made of a lithium-containing oxide, wherein the solvent of the non-aqueous electrolytic solution is an ester. Non-aqueous electrolyte secondary battery containing methyl propionate in ester. 2. The positive electrode active material is a composite oxide of lithium and cobalt, a composite oxide of lithium and nickel, a composite oxide of lithium and manganese, a composite oxide of lithium and iron, or cobalt of each of the above composite oxides. ,nickel,
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is one in which manganese and iron are partially replaced with another transition metal. 3. The nonaqueous electrolytic solution contains at least one of lithium hexafluorophosphate, lithium borofluoride, lithium perchlorate, lithium trifluoromethanesulfonate, and lithium hexafluoroarsenate as its solute. Item 3. The non-aqueous electrolyte secondary battery according to item 1 or 2.