JP3032338B2 - Non-aqueous electrolyte secondary battery - Google Patents

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 an improvement in cycle life and capacity characteristics at a low temperature of the battery.

[0002]

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

When a non-aqueous electrolyte battery is converted into a secondary battery, a high-capacity and high-voltage positive electrode active material is desired.
LiCoO ₂ , LiNi
O ₂ , LiFeO ₂ , and LiMn ₂ O ₄ based materials exhibiting a high voltage of 4 V are exemplified.

On the other hand, as a negative electrode material, a lithium alloy, a carbon material capable of occluding and releasing lithium ions, and the like, including metal lithium, are being studied. However, metallic lithium has a problem of short-circuit due to dendritic products (dendrites) during charging and discharging, and lithium alloy has problems such as collapse of electrodes due to expansion and contraction due to charging and discharging. Therefore, recently, a carbon material which does not cause these problems is regarded as promising as a negative electrode material of a lithium secondary battery. In general, when metal lithium is used as the negative electrode material, active dendrites generated on the negative electrode surface during charging react with the non-aqueous solvent to cause a partial solvent decomposition reaction, which often lowers the charging efficiency. Are known. To solve this problem, Japanese Patent Application Laid-Open No. 57-170463 focuses on the fact that ethylene carbonate is excellent in charging efficiency, and proposes to use a mixed solvent of ethylene carbonate and propylene carbonate. Further, Japanese Patent Laid-Open No. 3-5
In Japanese Patent No. 5770, in order to improve low-temperature characteristics of a battery, a mixed solvent of ethylene carbonate and diethyl carbonate is mixed with 2-methyltetrahydrofuran, 1,2-dimethoxyethane, 4-methyl-1,3-dioxolane, etc. It has been proposed to use as a solvent.

However, even when these systems are used, the charging efficiency is at most about 98 to 99%, and the charging efficiency has not yet been sufficiently improved. This is the same 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 electrolyte intercalate between layers of the carbon material, so that lithium dendrites are generated. Therefore, the decomposition reaction of the solvent on the negative electrode surface as described above should not occur.
However, the charging efficiency is actually less than 100%, and the same problem as when lithium or a lithium alloy is used for the negative electrode remains.

The present inventors believe that this phenomenon is not due to the decomposition reaction of the solution on the negative electrode surface as in the case where lithium metal is used for the negative electrode, but rather when lithium intercalates between the layers of the negative electrode carbon material. It is thought that it is absorbed not only by lithium but also by a solvent in which lithium is coordinated between the layers, and at this time, a part of the solvent causes a decomposition reaction. 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 entrance between the layers of the negative electrode material.

In general, a requirement for an excellent solvent for an electrolyte solution of a lithium battery is that the electrolyte has a large dielectric constant, that is, a large amount of an inorganic salt as a solute can be dissolved. Propylene carbonate, ethylene carbonate, etc. are said to be excellent solvents satisfying this requirement, but all of them have a large molecular radius due to their cyclic structure. There is a problem that a decomposition reaction of the solvent sometimes occurs. In addition, since these solvents have a high viscosity, when used alone, the electrolyte has a high viscosity, which makes it difficult to perform high-rate charging and discharging, and has a disadvantage that the capacity at low temperatures is small. In particular, ethylene carbonate has a high freezing point of 36.4 ° C. and cannot be used alone.

On the other hand, chain carbonates are structurally
These solvents easily enter the layers of the carbon material and are unlikely to undergo a decomposition reaction during charging. However, on the contrary, these solvents have a disadvantage that the dielectric constant is relatively low and it is difficult to dissolve the solute inorganic salt.

Further, when these cyclic and chain carbonates are used as a mixture, the above-mentioned problems caused when each is used alone can be solved, and the charge / discharge characteristics of the battery at normal temperature can be improved. However, it is insufficient to improve the charge / discharge characteristics of the battery at low temperatures. Usually, in lithium batteries, a method of adding a solvent having a low freezing point and a low viscosity to the solvent in the electrolytic solution is used to improve low-temperature characteristics. At the time of charging, the decomposition reaction of the solvent as described above is accompanied.

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

Further, the present invention also aims to provide a solvent composition of preferred non-aqueous electrolyte solution for nonaqueous electrolyte secondary batteries.

[0013]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems and to achieve the above-mentioned object, the present invention provides a three-component system comprising ethylene carbonate as a cyclic ester, diethyl carbonate as a chain ester, and methyl propionate. A system mixed solvent is used as a solvent for the electrolytic solution. In particular, the proportion of ethylene carbonate in the total solvent is 20% to 50% by volume, and the proportion of methyl propionate in the chain ester of the solvent component is 25% to 75% by volume.
The following provides an excellent electrolyte solution for a non-aqueous electrolyte secondary battery.

[0014]

[Effect] Ethylene carbonate in the electrolytic solvent is effective in increasing the conductivity of the electrolytic solution by dissolving a large amount of an inorganic salt as a solute, and diethyl carbonate coordinates lithium easily during charging of a battery to easily form a carbonate. Since it can enter between layers of the material, decomposition of the solvent can be suppressed. Further, by mixing methyl propionate having a low freezing point and low viscosity with these, the freezing point and viscosity of the electrolytic solution are reduced, and 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 is a longitudinal sectional view of a cylindrical battery. In the figure, reference numeral 1 denotes a positive electrode, and the active material is Li
A mixture of carbon black as a conductive material of CoO ₂ and an aqueous dispersion of polytetrafluoroethylene in a weight ratio of 100: 3: 10 as a binder is applied to both sides of an aluminum foil and dried. After rolling, it is cut into a predetermined size. In this case, two 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 mainly composed of a carbonaceous material, and which has a weight ratio of 100: 5 with an acrylic binder.
Are coated on both sides of a nickel foil, dried, rolled, and then cut to a predetermined size. Also in this case, a negative electrode lead plate made of nickel 4 is spot-welded. 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 entirely spirally wound to form an electrode plate group. Insulating plates 6 and 7 made of polypropylene are arranged on the upper and lower ends of the electrode plate group, respectively, and inserted into a case 8 plated with nickel on iron. Then, the positive electrode lead 2 is attached to a sealing plate 10 made of titanium,
After spot welding the negative electrode lead 4 to the bottom of the case 8, a predetermined amount of electrolyte is injected into the case, and the battery is sealed with a sealing plate 10 via a gasket 9 to obtain a completed battery. The dimensions of this battery are 14 mm in diameter and 50 mm in height.
Reference numeral 11 denotes a positive electrode terminal of the battery, and the negative electrode terminal is also used by the battery case 8.

Ethylene carbonate (hereinafter referred to as EC) and diethyl carbonate (hereinafter referred to as DE)
C) and methyl propionate (hereinafter referred to as MP) were combined to prepare the following three types of mixed solvent systems (all by volume ). went. In addition, lithium hexafluorophosphate was used as a solute of the electrolytic solution, and each was adjusted to have a concentration of 1 mol / l.

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

The voltage and current conditions are as follows: charge / discharge current 100 m
A, the charge end voltage was 4.2 V, and the discharge end voltage was 3.0 V. First, after the initial 10 cycles of charge / discharge were performed at 20 ° C., the test was stopped in the charged state, the temperature was changed to −10 ° C., and the low-temperature characteristics were evaluated based on the amount of discharge. Thereafter, the temperature was returned to 20 ° C., and charge / discharge was repeated. When the discharge capacity was reduced to 50% of the initial value, the test was terminated, and the number of cycles was defined as the cycle life.

However, the solvent in the electrolytic solution of the battery C was EC alone, and the freezing point of the EC was 36.4 ° C.
Only at 40 ° C. was charged and discharged.

FIG. 2 shows the cycle life characteristics of the batteries A to C, and FIG. 3 shows the low temperature characteristics. According to FIG. 2, BAC was obtained in the order of the cycle life characteristics. This is because during charging, lithium ions intercalate into the interlayer of the carbon material at the negative electrode, but the solvent molecules coordinated to the lithium ions are also drawn between the layers at that time, so a solvent having a cyclic structure and a large molecule It is thought to be partially disassembled. It is considered that the battery C (EC 100%) having a large content of the solvent having a cyclic structure has poor characteristics. The reason that the life of battery A is shorter than that of battery B is that DEC
This is considered to be because the voltage is stable at a higher voltage than P and the degree of decomposition accompanying the cycle is small.

Next, FIG. 3 shows that AB-
It became C. Since the battery C used EC having a high freezing point alone, no discharge was possible at -10 ° C. Battery B
It is also considered that the electrolyte mixture is considerably thickened due to the high mixing ratio of EC, so that the polarization becomes large and the discharge capacity is small.

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

From the above results, it was found that low-temperature characteristics could be improved without impairing cycle life characteristics by using a three-component mixed system of EC, DEC and MP as a solvent for the electrolytic solution.

Next, a second embodiment will be described. (Example 2) EC used in Example 1 as a solvent for the electrolytic solution
A prototype of the above cylindrical battery was produced for the following five types of mixed solvent systems prepared by combining three components, DEC and MP. The solute of the electrolytic solution was also adjusted to 1 mol / l using lithium hexafluorophosphate as in Example 1.

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

FIG. 4 shows the cycle life characteristics of the batteries D to H, and FIG. 5 shows the low-temperature characteristics. From FIG. 4, D-E-F-G-H is obtained in the order of good cycle life characteristics, and E is a cyclic ester.
As the mixing ratio of C increases, the cycle characteristics deteriorate,
In particular, when H EC was added in an amount of 60% or more of the whole solvent, the properties were particularly poor. Next, according to FIG. 5, the low temperature characteristics are E, F, and G.
Was good, and D and H were bad. In the case of H, since the mixture ratio of EC was high, the electrolyte solution was frozen at a low temperature, and no discharge was possible. On the other hand, D is bad because E has a high dielectric constant.
It is considered that, because the mixing ratio of C is small, the ability to dissolve a predetermined amount of solute at a low temperature is lost, solute is precipitated, the liquid resistance is increased, and the polarization is increased.

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) EC, DEC, and MP were used as the solvent for the electrolytic solution in the same manner as in Example 2.
The prototype of the above cylindrical battery was produced for the following six types of mixed solvent systems prepared by combining the three components described above. Solute of the electrolyte solution even using the same lithium hexafluorophosphate as in Examples 1 and 2 was adjusted to respectively a concentration of 1 mol / l.

Battery I ... EC: DEC: MP = 40: 12: 48 Battery J ... EC: DEC: MP = 40: 15: 45 Battery K ... EC: DEC: MP = 40: 24: 36 Battery L ... EC: DEC: MP = 40: 36: 24 Battery M ... EC: DEC: MP = 40: 45: 15 Battery N ... EC: DEC: MP = 40: 48: 12 Structural conditions other than the above electrolytic solution 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. FIG. 6 shows that the cycle life characteristics are NMLKJJI in descending order, and that the cycle life characteristics deteriorate as the mixing ratio of MP increases. This is because, apart from the solvent decomposition reaction that occurs at the negative electrode when the battery is charged as described above, the solvent is oxidatively decomposed because a compound having a high potential is used at the positive electrode. Thus, the carbonates were stable at a higher potential than the other chain esters, and thus the cycle deterioration was larger as the ratio of DEC in the chain esters was smaller, that is, the ratio of MP was larger. 1 showed particularly poor properties, the above MP decomposition reaction occurred remarkably when MP was contained in 80% or more of the chain ester. Therefore, the mixing ratio of MP in the chain ester was 75%. The following results were found to be appropriate. Next, FIG. 7 shows that IJKLMN is in descending order of the low-temperature characteristics. As the mixing ratio of MP increases, the viscosity of the solvent decreases and the discharge capacity at low temperatures increases. Further, the optimum mixing ratio was 25% or more, and below that, no significant effect was obtained. Considering the cycle life characteristics and the low temperature characteristics as described above, the optimum mixing ratio of MP is 25 to 25 in the chain ester.
It can be said that it is 75%.

When the results of the above three examples are combined, EC and DE are used as the solvent for the electrolyte of a lithium secondary battery using a lithium composite oxide having a high potential as the positive electrode and a carbon material as the negative electrode.
When a three-component mixed system of C and MP is used, good cycle life characteristics and low-temperature characteristics are exhibited. The optimum mixing ratio is such that EC is 20 to 50% of the whole solvent and MP is 25 to 75% in the chain ester. %.

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

In the present embodiment, lithium hexafluorophosphate was used as a solute of the electrolytic solution. However, other lithium-containing salts, for example, lithium borofluoride, lithium perchlorate, lithium trifluoromethanesulfonate, lithium hexafuccinate, etc. Almost the same result was obtained with the above.

[0035]

As is apparent from the above description, according to the present invention, a ternary mixed solvent of ethylene carbonate, diethyl carbonate and methyl propionate is used as the solvent for the electrolytic solution, and the volume ratio of ethylene carbonate is adjusted to the whole solvent. By setting the volume ratio of methyl propionate to 25 to 75% of the chain ester, a non-aqueous electrolyte secondary battery having excellent cycle life characteristics and low-temperature characteristics can be provided. .

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a cycle life at 20 ° C. of the battery in Example 1.

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

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

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

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

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

[Explanation of symbols]

 REFERENCE SIGNS LIST 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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takayuki Kawahara 1, Tohocho, Yokkaichi-shi, Mie Mitsubishi Yuka Co., Ltd. (72) Inventor Katsuaki Hasegawa 1, Tohocho, Yokkaichi-shi, Mie Mitsubishi Yuka Stock (56) References JP-A-3-295178 (JP, A) JP-A-3-55770 (JP, A) JP-A-5-13105 (JP, A) (58) Fields surveyed ( Int.Cl. ⁷ , DB name) H01M 10/40

Claims (5)

(57) [Claims] 1. A negative electrode comprising a carbon material capable of inserting and extracting lithium ions, a non-aqueous electrolyte, and a positive electrode comprising a lithium-containing oxide, wherein the solvent of the non-aqueous electrolyte is ethylene carbonate, diethyl carbonate and Non-aqueous electrolyte secondary battery comprising methyl 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 from 20% to 50% of the whole solvent. 3. The volume ratio of methyl propionate in the chain ester in the solvent component of the electrolytic solution is not less than 25% and not more than 75%.
The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein: 4. 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; The nonaqueous 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 a solute. Items 1-4
The non-aqueous electrolyte secondary battery according to any one of the above.