JPH0574488A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous electrolyte secondary battery which is excellent at cycle life characteristic and low temperature characteristics by improving the solvent of electrolyte. CONSTITUTION:A nonaqueous electrolyte secondary battery includes a negative electrode 3 made by a carbonaceous material which can store and release lithium ions, nonaqueous electrolyte, and a positive electrode 1 made by an oxide containing lithium, and the solvent of the nonaqueous electrolyte is formed by mixing of ethylene carbonate, diethyl carbonate and propionic acid ethyl. The mixing ratio of the ethylene carbonate is 20 to 50% of the whole solvent and the mixing ratio of the propionic acid ethyl is 25 to 75% of chain ester. The nonaqueous electrolyte secondary battery excellent in cycle life and low temperature characteristics is thus obtained.

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 improvement of cycle life and capacity characteristics at low temperature of this battery.

[0002]

2. Description of the Related Art In recent years, portable 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 devices. From this point of view, 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.
To meet this demand, LiCoO ₂ , LiNi
Examples thereof include O ₂ , LiFeO ₂ , and LiMn ₂ O ₄ -based materials that exhibit a high voltage of 4V.

On the other hand, as negative electrode materials, lithium metal, lithium alloys, carbon materials capable of absorbing and releasing lithium ions, and the like 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. Generally, when metallic lithium is used as the negative electrode material, active dendrites generated on the negative electrode surface during charging react with a non-aqueous solvent to cause a partial solvent decomposition reaction, which often lowers charging efficiency. Are known. As a solution to this, JP-A-57-170463 proposes the use of a mixed solvent of ethylene carbonate and propylene carbonate, paying attention to the fact that ethylene carbonate is excellent in charging efficiency. Furthermore, JP-A-3-5
In 5770, 2-methyltetrahydrofuran, 1,2-dimethoxyethane, and 4-methyl-1,3-dioxolane are mixed in a mixed solvent of ethylene carbonate and diethyl carbonate in order to improve low-temperature characteristics of a battery, and a non-aqueous electrolyte solution It has been proposed to use it as a solvent.

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.

[0006]

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 solution 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 is thought that this is due to the fact that not only lithium but also lithium is coordinated with the solvent in which it is coordinated, and it causes a decomposition reaction of some of the solvent. 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. Propylene carbonate, ethylene carbonate, etc. are said to be excellent solvents that satisfy this requirement. However, since all of them have a large molecular radius due to their cyclic structure, when a carbon material is used for the negative electrode, they are charged as described above. It sometimes has a problem that it involves a decomposition reaction of the solvent. Further, since these solvents are highly viscous, when used alone, they have the drawbacks 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.

On the other hand, the chain carbonates are structurally
The carbon material easily enters between the layers and the decomposition reaction at the time of charging does not easily occur. On the contrary, these solvents have a disadvantage that the dielectric constant is relatively low and the solute inorganic salt is difficult to dissolve.

When these cyclic and chain carbonates are mixed and used, the above-mentioned problems that occur when they are used alone are solved, and the charge / discharge characteristics of the battery at room temperature can be improved. However, it is insufficient to improve the charge / discharge characteristics of the battery at low temperature. Usually, in a lithium battery, a method of adding a solvent having a low freezing point and a low viscosity to the solvent in the electrolytic solution in order to improve low-temperature characteristics is adopted, but in this case, when a solvent having a cyclic structure such as cyclic ether is used, At the time of charging, the above-described decomposition reaction of the solvent is involved.

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

Another object of the present invention is to provide a solvent composition of the non-aqueous electrolyte solution which is preferable for the non-aqueous electrolyte secondary battery.

[0013]

In order to solve the above-mentioned problems and to achieve the above-mentioned object, the present invention comprises three components of ethylene carbonate which is a cyclic ester, diethyl carbonate which is a chain ester and ethyl propionate. The system mixed solvent is used as the solvent of the electrolytic solution. In particular, the ratio of ethylene carbonate in the total solvent is 20% to 50% by volume, and the ratio of ethyl propionate in the chain ester of the solvent component is 25% to 75% by volume.
The following provides an excellent electrolytic solution for a non-aqueous electrolytic solution secondary battery.

[0014]

[Function] The ethylene carbonate in the electrolyte solvent is effective in increasing the conductivity of the electrolyte by dissolving a large amount of the inorganic salt that is a solute, and the diethyl carbonate facilitates coordination of lithium during battery charging. Since it can enter between the layers of the carbon material, the decomposition of the solvent can be suppressed. By further mixing ethyl propionate having a low freezing point and low viscosity with these, the freezing point and viscosity of the electrolytic solution are lowered,
As a result, excellent low temperature characteristics are exhibited.

[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
A mixture of carbon black as a conductive material of CoO ₂ and an aqueous dispersion of polytetrafluoroethylene at a weight ratio of 100: 3: 10 as a binder is 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 to 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 spirally wound 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, and the plates are 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 dimensions of this battery are 14 mm in diameter and 50 mm in height.
Reference numeral 11 is a positive electrode terminal of the battery, and the battery case 8 also serves as a negative electrode terminal.

As a solvent for the electrolytic solution, ethylene carbonate (hereinafter referred to as EC), diethyl carbonate (hereinafter referred to as DE)
C), ethyl propionate (hereinafter referred to as EP), and prepared three types of mixed solvent systems (all weight ratios) shown below. went. Lithium hexafluorophosphate was used as the solute of the electrolytic solution, and the concentration was adjusted to 1 mol / l.

Battery A ... EC: DEC: EP = 3: 3: 4 Battery B ... EC: DEC: EP = 5: 5: 0 Battery C ... EC: DEC: EP = 10: 0: 0 Evaluation Battery characteristics are cycle life characteristics and low temperature characteristics.

The voltage and current conditions are as follows: charge / discharge current 100 mA,
The final charge voltage was 4.2V and the final discharge voltage was 3.0V.
First, after performing initial 10 cycles of charge and discharge 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 amount. Then, the temperature was returned to 20 ° C. and charging / discharging was repeated. The test was terminated when the discharge capacity deteriorated to 50% of the initial value, and the number of cycles was taken as the cycle life.

However, since the solvent in the electrolytic solution of Battery C is EC alone and the freezing point of EC is 36.4 ° C., Battery C is
It was charged and discharged only at 40 ° C.

FIG. 2 shows the cycle life characteristics of the batteries A to C, and FIG. 3 shows the low temperature characteristics thereof. From FIG. 2, B-A-C are shown in descending order of cycle life characteristics. During charging, the intercalated intercalated helium ions of the carbon material at the negative electrode intercalate, but at that time, solvent molecules coordinated to the lithium ions are also drawn in between the layers, so that the solvent has a cyclic structure and large solvent It is thought to be partially decomposed. It is considered that this is the reason why the battery C (EC 100%) having a large content of the solvent having a cyclic structure has poor characteristics. In addition, the battery life of battery A is shorter than that of battery B.
It is considered that this is because it is unstable at a higher voltage than C and the degree of decomposition accompanying the cycle is small.

Next, as shown in FIG.
It became C. Battery C could not be discharged at −10 ° C. at all because EC having a high freezing point was used alone. Also, the battery B
It is considered that since the mixing ratio of EC is high, the electrolytic solution is considerably thickened, so that the polarization is increased and the discharge capacity is small.

On the other hand, when low-viscosity EP was added to the battery A, good low-temperature characteristics were exhibited. Therefore, it was found that adding a low-viscosity solvent is effective in improving the low-temperature characteristics. ..

From the above results, it was found that the low temperature characteristics can be improved without impairing the cycle life characteristics by using the ternary mixture system of EC, DEC and EP as the solvent of the electrolytic solution.

Next, a second embodiment will be described. (Example 2) EC used in Example 1 as a solvent for an electrolytic solution
The above-mentioned cylindrical battery was prototyped for the following five types of mixed solvent systems prepared by combining the three components of DEC and EP. As the solute of the electrolytic solution, lithium hexafluorophosphate was used as in Example 1, and the concentration was adjusted to 1 mol / l.

Battery D ... EC: DEC: EP = 10: 45: 45 Battery E ... EC: DEC: EP = 20: 40: 40 Battery F ... EC: DEC: EP = 40: 30: 30 Battery G EC: DEC: EP = 50: 25: 25 Battery H ... EC: DEC: EP = 60: 20: 20 The constitutional conditions and test conditions other than the above electrolytic solution were the same as in Example 1.

The cycle life characteristics of the batteries D to H are shown in FIG. 4, and the low temperature characteristics are shown in FIG. From FIG. 4, D-E-F-G-H is obtained in the order of good cycle life characteristics, and is a cyclic ester EC.
The cycle characteristics deteriorated as the mixing ratio of H was increased, and the characteristics were particularly poor when EC of H was added to 60% or more of the entire solvent. Next, as shown in FIG. 5, the low temperature characteristics are good in E, F and G and bad in D and H. Since H had a high EC mixing ratio, the electrolytic solution was frozen at a low temperature and no discharge was possible. On the other hand, the reason why D is bad is that the mixing ratio of EC having a high dielectric constant is small, so that the ability to dissolve a predetermined amount of solute at low temperature is lost, solute precipitation occurs, the liquid resistance increases, and the polarization increases. Conceivable.

Therefore, the mixing ratio of EC is 20 to 20% of the whole solvent.
About 50% is considered to be an appropriate range.

Next, a third embodiment will be described. (Example 3) The same as in Example 2 was prepared by combining three components of EC, DEC, and EP as a solvent for the electrolytic solution, and prepared as follows 6
The above-mentioned cylindrical battery was prototyped for various mixed solvent systems. As the solute of the electrolytic solution, lithium hexafluorophosphate was used as in Examples 1 and 2, and the concentration was adjusted to 1 mol / l.

Battery I ... EC: DEC: EP = 40: 12: 48 Battery J ... EC: DEC: EP = 40: 15: 45 Battery K ... EC: DEC: EP = 40: 24: 36 Battery L EC: DEC: EP = 40: 36: 24 Battery M ... EC: DEC: EP = 40: 45: 15 Battery N ... EC: DEC: EP = 40: 48: 12 Configuration conditions other than the above electrolyte The test conditions were the same as in Examples 1 and 2.

FIG. 6 shows the cycle life characteristics of the batteries I to N, and FIG. 7 shows the low temperature characteristics thereof. From FIG. 6, N-M-L-K-J-I was obtained in descending order of cycle life characteristics, and the cycle life characteristics deteriorated as the EP mixing ratio increased. This is because the solvent is oxidatively decomposed because a compound showing a high potential is used for the positive electrode in addition to the solvent decomposition reaction that takes place in the negative electrode during charging of the battery as described above. In contrast, the carbonates were stable at a higher potential than other chain esters, and therefore, the smaller the DEC ratio in the chain esters, that is, the larger the EP ratio, the greater the cycle deterioration. Since I has a particularly bad characteristic, the decomposition reaction of EP remarkably occurs when EP is contained in an amount of 80% or more in the chain ester, and therefore the mixing ratio of EP in the chain ester is 75% or less. Is suitable. Next, from FIG. 7, I-J-K-L-M-N was obtained in the order of good low-temperature characteristics, and the higher the mixing ratio of EP, the lower the viscosity of the solvent and the larger the discharge capacity at low temperature. The optimum mixing ratio is 2
If it is 5% or more and less than 5%, the effect is not obtained so much. Considering the two points of cycle life characteristics and low temperature characteristics, the optimum mixing ratio of EP is 25 to 75 in the chain ester.
Can be said to be%.

Summarizing the results of the above three examples, EC and DE were used as the solvent of the electrolyte solution of the lithium secondary battery in which the positive electrode is the lithium composite oxide showing a high potential and the negative electrode is the carbon material.
When using a three-component mixed system of C and EP, good cycle life characteristics and low temperature characteristics are exhibited, and the optimum mixing ratio is 20 to 50% of the whole EC solvent, and EP is 25 to 75% of the chain ester. I found out.

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, 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 replaced with other transition metals.

In this embodiment, lithium hexafluorophosphate is used as the solute of the electrolytic solution. However, other lithium-containing salts such as lithium borofluoride, lithium perchlorate, lithium trifluoromethanesulfonate, and hexafluoroarsenic are used. Almost similar results were obtained with lithium oxide and the like.

[0035]

As is apparent from the above description, according to the present invention, a three-component system mixed solvent of ethylene carbonate, diethyl carbonate and ethyl propionate is used as a solvent of an electrolytic solution, and the volume ratio of ethylene carbonate is set to the whole solvent. It is possible to provide a non-aqueous electrolyte secondary battery having excellent cycle life characteristics and low-temperature characteristics by adjusting the volume ratio of ethyl propionate to 25 to 75% of the chain ester in the chain ester. ..

[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 1 at 20 ° C.

FIG. 3 is a graph showing changes in discharge voltage at −10 ° C. of the battery in Example 1.

FIG. 4 is a diagram showing the cycle life of the battery in Example 2 at 20 ° C.

FIG. 5 is a graph showing changes in discharge voltage at −10 ° C. of the battery in Example 2.

FIG. 6 is a diagram showing the cycle life of the battery in Example 3 at 20 ° C.

FIG. 7 is a graph showing changes in discharge voltage at −10 ° C. of the battery in Example 3.

[Explanation of symbols]

 1 Positive Electrode 2 Positive Electrode Lead Plate 3 Negative Electrode 4 Negative Electrode Lead Plate 5 Separator 6 Upper Insulation Plate 7 Lower Insulation Plate 8 Case 9 Gasket 10 Sealing Plate 11 Positive Electrode Terminal

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takayuki Kawahara 1 Toho Town, Yokkaichi City, Mie Prefecture Yokkaichi Research Institute, Mitsubishi Petrochemical Co., Ltd. (72) Inventor Katsuaki Hasegawa 1 Toho Town, Yokkaichi City, Mie Prefecture Mitsubishi Petrochemical Incorporated company Yokkaichi Research Institute

Claims (5)

[Claims] 1. 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. The solvent of the non-aqueous electrolyte solution is ethylene carbonate and diethyl carbonate. Non-aqueous electrolyte secondary battery made of ethyl propionate. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the volume ratio of ethylene carbonate in the solvent component of the electrolyte is 20% or more and 50% or less of the entire solvent. 3. The volume ratio of ethyl propionate in the chain ester in the solvent component of the electrolytic solution is 25% or more and 75% or more.
The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein: 4. 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 the above composite oxides, respectively. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein nickel, manganese, and iron are partially substituted with another transition metal. 5. The non-aqueous electrolyte contains at least one of lithium hexafluorophosphate, lithium borofluoride, lithium perchlorate, lithium trifluoromethanesulfonate, and lithium hexafluoroarsenate as its solute. Items 1 to 4
The non-aqueous electrolyte secondary battery according to any one of 1.