JPH0613109A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To achieve a high energy density, a long life and high reliability in a wide range of temperature for use by using mixed solvent of propylene carbonate(PC) and methyl ethyl carbonate(MEC) as nonaqueous electrolyte. CONSTITUTION:A nonaqueous secondary battery is formed with a negative plate 1 of carbon material, a positive plate 2 of lithium composite oxide. As nonaqueous solvent, PC/MEC mixed low-viscocity solvent is used so that the nonaqueous electrolyte secondary battery shows a high energy density, a long life and high reliability in a wide range of temperature for use without increasing its inner pressure under high temperature circumstances and solidifying its electrolyte under low temperature circumstances.

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 a non-aqueous electrolyte secondary battery using a lithium composite oxide as a positive electrode and a carbonaceous material as a negative electrode.

[0002]

2. Description of the Related Art Recently, a VTR with a built-in camera, a mobile phone,
New portable electronic devices such as laptop computers have appeared one after another, and their size and weight have been further reduced.
Along with this, a secondary battery has been spotlighted as a portable power source, and active research and development has been conducted to obtain higher energy density.

Under such circumstances, a lithium ion secondary battery has been proposed and put into practical use as a secondary battery having a higher energy density than that of an aqueous electrolyte secondary battery such as a lead battery and a nickel cadmium battery.

By the way, as an organic electrolytic solution for a lithium secondary battery, since a low-viscosity solvent alone has a low charge / discharge efficiency, it is indispensable to mix with a high dielectric constant solvent such as PC (propylene carbonate). In addition, when PC is mixed, there is an advantage that higher electric conductivity can be obtained as compared with a single component system containing only a low viscosity solvent. From this,
Conventionally, a mixed solvent formed by mixing PC and DME (1,2-dimethoxyethane) has been used as an organic electrolytic solution for a lithium secondary battery.

However, since the non-aqueous electrolyte secondary battery using a mixed solvent of PC and DME has a low boiling point of DME, the internal pressure detection type current interruption device malfunctions when the operating environment temperature rises to about 100 ° C. However, a situation occurs in which charging and discharging cannot be performed. For this reason, it cannot be used in a car, for example, in the summer. In addition, since DME has low stability with respect to a lithium composite oxide, if the battery is stored in a charged state, irreversible capacity deterioration occurs.

As a low-viscosity solvent other than DME, JP-A-2-148665 proposes a low-molecular-weight carbonic acid ester having a carbon number n of 1 to 3, and the carbonic acid ester alone is used as an organic solvent for a lithium secondary battery. When used as a solvent,
It is stated that the cycle life of the lithium metal negative electrode is improved. However, in this case, the cycle life is 25
The cycle life was about 0, which was considerably lower than the cycle life of the existing nickel-cadmium secondary battery of about 500 times, and was not satisfactory as a practical battery.

On the other hand, when the above-mentioned low molecular weight carbonic acid ester is mixed with PC, the cycle characteristics are improved, and especially in a non-aqueous electrolyte secondary battery using a carbonaceous material as the negative electrode, about 1000 times or more. Cycle life is obtained.

For example, among low molecular weight carbonic acid esters, a mixed solvent of DEC (diethyl carbonate) and PC having a high boiling point is unlikely to cause an increase in internal pressure of the battery due to an increase in environmental temperature as compared with the case where a mixed solvent of DME and PC is used. The stability with respect to the positive electrode is also improved. However, there is also a problem that the capacity retention ratio, which is indicated by the ratio of the capacity before and after the charge / discharge cycle, is relatively small and the cycle life is shortened.

A mixed solvent of DMC (dimethyl carbonate) and PC has also been investigated, and it has been confirmed that the use of this mixed solvent improves the cycle life and stability to the positive electrode. However, since the mixed solvent of DMC and PC has a low boiling point and a high freezing point, it has a drawback that the operating temperature range of the battery is narrowed.

[0010]

As described above, in the conventional non-aqueous electrolyte secondary battery, since the organic solvent is not adequately suitable, the operating temperature is limited and the sufficient cycle life is obtained. There is an inconvenience that there is no.

Therefore, the present invention has been proposed in view of such conventional circumstances, and a non-aqueous electrolyte secondary battery which can obtain high energy density, long life and high reliability in a wide operating temperature range. The purpose is to provide.

[0012]

In order to achieve the above object, the inventors of the present invention searched for an appropriate organic solvent as a low-viscosity solvent for a non-aqueous electrolyte secondary battery, and found that one methyl group of DMC was used. MEC (methyl ethyl carbonate) was found in which the groups were replaced by ethyl groups. MEC has a boiling point and a freezing point almost in between those of DMC and DEC, and is superior in stability to positive and negative electrodes than DEC. By using it in combination with PC, MEC can be used as a non-aqueous solvent for non-aqueous electrolyte secondary batteries. It will be suitable.

The non-aqueous electrolyte secondary battery of the present invention has been completed based on such findings. That is, the non-aqueous electrolyte secondary battery of the present invention is doped with lithium ions.
An anode made of a carbonaceous material that can be dedoped, and Li _X MO
₂ (where M represents at least one of cobalt, nickel and manganese), and a non-aqueous electrolyte secondary battery comprising an electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent, The non-aqueous solvent is a mixed solvent of propylene carbonate and methyl ethyl carbonate.

Further, the non-aqueous solvent is a mixed solvent in which propylene carbonate and methyl ethyl carbonate are mixed in a volume ratio of 7: 3 to 3: 7. Furthermore, LiP is used as an electrolyte in a non-aqueous solvent.
It is characterized in that F ₆ is dissolved at a concentration of 0.6 to 1.8 mol / l.

The positive electrode material used in the non-aqueous electrolyte secondary battery of the present invention has a general formula of Li _X MO ₂ (where M is C
represents at least one of o, Ni, and Mn. ) And a lithium compound metal oxide represented by the above) or an intercalation compound containing Li. Among them, particularly when LiCoO ₂ is used, a high energy density is exhibited.

As the carbonaceous material for the negative electrode, any of those used for this type of secondary battery can be used, but the carbonaceous materials listed below are particularly preferable.

First, there is a carbon material obtained by carbonizing an organic material by a method such as firing.

As the organic material as a starting material, phenol resin, acrylic resin, vinyl halide resin, polyimide resin, polyamideimide resin, polyamide resin,
Conjugated resins such as polyacetylene and poly (p-phenylene), cellulose and its derivatives, and any organic polymer compounds can be used.

In addition, condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphene and pentacene,
Other derivatives (for example, these carboxylic acids, carboxylic acid anhydrides, carboxylic acid imides, etc.), various pitches containing a mixture of the respective compounds as a main component, acenaphthylene, indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole, Fused heterocyclic compounds such as acridine, phenazine and phenanthridine, and their derivatives can also be used.

Furan resins which are homopolymers or copolymers of furfuryl alcohol or furfural are also particularly suitable. The carbonaceous material obtained by carbonizing this furan resin has a (002) plane spacing of 3.70 Å or more, a true density of 1.70 g / cc or less, and a DTA (differential thermal analysis) of 7
It does not have an oxidation exothermic peak at 00 ° C. or higher and shows very good characteristics as a negative electrode material for batteries.

The temperature for firing these organic materials is as follows:
Depends on the starting material, usually 500-3000 ° C
It is said that

Alternatively, a petroleum pitch having a specific H / C atomic ratio to which a functional group containing oxygen is introduced (so-called oxygen cross-linking) exhibits excellent characteristics when carbonized, like the furan resin. Therefore, it can be used as the organic material. The petroleum pitch is distilled from tars obtained by high-temperature thermal decomposition of coal tar, ethylene bottom oil, crude oil, asphalt, etc. (vacuum distillation,
It can be obtained by operations such as atmospheric distillation, steam distillation), thermal polycondensation, extraction, and chemical polycondensation.

At this time, the H / C atomic ratio of petroleum pitch is important, and it is necessary to set the H / C atomic ratio to 0.6 to 0.8 in order to obtain non-graphitizable carbon.

The specific means for introducing a functional group containing oxygen into these petroleum pitches is not limited, but for example nitric acid,
Wet method using an aqueous solution of mixed acid, sulfuric acid, hypochlorous acid, etc., or dry method using oxidizing gas (air, oxygen), and reaction with solid reagents such as sulfur, ammonium nitrate, ammonium persulfate, ferric chloride, etc. Used. For example, when oxygen is introduced into the petroleum pitch by the above method, the final carbonaceous material is obtained in the solid state without melting during the carbonization process (400 ° C or higher), which is the process of forming non-graphitizable carbon. Similar to.

The petroleum pitch having oxygen-containing functional groups introduced therein is carbonized by the above-mentioned method to be used as a negative electrode material, and the (002) plane spacing is 3.70Å regardless of the conditions for carbonization.
Above, true density 1.70g / cc or less and DTA 700
If a carbonaceous material satisfying the characteristic of not having an oxidation exothermic peak above 0 ° C. is obtained, a large amount of lithium doped per unit weight can be obtained. For example, when the oxygen content of the precursor obtained by oxygen-crosslinking petroleum pitch is 10% by weight or more, the interplanar spacing of (002) planes can be 3.70 Å or more. Therefore,
The oxygen content of the precursor is preferably 10% by weight or more, and practically in the range of 10 to 20% by weight.

When a carbonaceous material is obtained by using the above raw material organic material, for example, after carbonizing at 300 to 700 ° C. in a nitrogen stream, the temperature rising rate is 1 to 20 ° C. per minute in the nitrogen stream, and the reached temperature is 900. ~ 1300 ℃, holding time at ultimate temperature 0-5
It suffices to bake under conditions of time. Of course, in some cases, the carbonization operation may be omitted.

Furthermore, by adding a phosphorus compound or a boron compound when carbonizing the furan resin or petroleum pitch, a special negative electrode compound having a large lithium doping amount can be used.

Examples of the phosphorus compound include phosphorus oxides such as phosphorus pentoxide, oxo acids such as orthophosphoric acid and salts thereof, and the like. Phosphorus oxides and phosphoric acids are preferable from the viewpoint of easy handling. is there. The amount of the phosphorus compound added is 0.2 to 30 in terms of phosphorus with respect to the organic material or the carbonaceous material.
% By weight, preferably 0.5 to 15% by weight, and the proportion of phosphorus remaining in the negative electrode material is 0.2 to 9.0% by weight, preferably 0.3 to 5% by weight.

As the boron compound, boron oxide or boric acid can be added in the form of an aqueous solution. The amount of the boron compound added is 0.2 to 30% by weight, preferably 0.5 to 15% by weight, in terms of boron based on the organic material or the carbonaceous material, and the proportion of boron remaining in the negative electrode material is 0. 0.2 to 9.0% by weight, preferably 0.3 to 5
Weight%

The non-aqueous electrolyte secondary battery of the present invention is housed in a battery can together with the above-mentioned negative electrode made of a negative electrode material and a positive electrode made of a positive electrode material together with an electrolytic solution obtained by dissolving an electrolyte in a non-aqueous solvent. Consists of

In the present invention, a mixed solvent composed of MEC, which is a low viscosity solvent, and PC, which is a high dielectric constant solvent, is used as the non-aqueous solvent in which the electrolyte is dissolved. MEC and PC
The mixed solvent consisting of the above has an appropriate boiling point and freezing point and is highly stable with respect to the positive electrode. Therefore, by using such a mixed solvent, high energy density and long cycle life can be obtained in a wide temperature range of use.

In the mixed solvent consisting of MEC and PC, PC and MEC have a PC: MEC (volume ratio) of 7:
It is preferable that they are mixed so as to be 3 to 3: 7. When the mixing ratio of PC and MEC is out of the above range, the conductivity of the electrolytic solution becomes low, and the battery characteristics are likely to be insufficient.

An electrolyte is dissolved in the above non-aqueous solvent, and as the electrolyte, LiPF ₆ , LiClO ₄ , L
iAsF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH
₃ SO ₃ Li, CF ₃ SO ₃ Li, LiCl, LiBr
Etc. are used. There is an appropriate range for the concentration of these electrolytes dissolved in the electrolytic solution. For example, in the case of LiPF ₆ ,
It is desirable to dissolve it in the electrolytic solution at a concentration of 0.6 to 1.8 mol / l. When the concentration of LiPF ₆ dissolved in the electrolytic solution is out of the above range, the electric conductivity of the electrolytic solution may be insufficient and the charge / discharge efficiency may be insufficient.

[0034]

In the non-aqueous electrolyte secondary battery of the present invention, PC is used as the high dielectric constant solvent of the electrolyte and ME is used as the low viscosity solvent.
Use C. Here, ME used as a low-viscosity solvent
C has a boiling point and a freezing point almost in the middle of those of DMC and DEC, is suitable as a low-viscosity solvent for an electrolytic solution, and is superior in stability to positive and negative electrodes to DEC. Therefore, the non-aqueous electrolyte secondary battery, the battery internal pressure hardly rises even when used in a high temperature environment, and the electrolyte does not freeze even when used in a low temperature environment, and has a wide operating temperature range. In, high energy density and long life can be obtained.

When the mixed solvent of MEC and PC is used as the non-aqueous solvent, the conductivity of the electrolytic solution is improved if the mixing ratio of MEC and PC and the dissolved concentration of the electrolyte are optimized.
The non-aqueous electrolyte secondary battery has more excellent charge / discharge efficiency.

[0036]

EXAMPLES The present invention will be described below with reference to concrete examples according to experimental results.

Example 1 In this example, LiPF ₆ was dissolved in a mixed solvent in which PC and MEC were mixed at a volume ratio of 1: 1 as an electrolytic solution, and a non-graphitizable carbon material was used as a negative electrode material. It is an example of a non-aqueous electrolyte secondary battery using LiCoO ₂ as a positive electrode material. The structure of the non-aqueous electrolyte secondary battery produced in this example is shown in FIG.

First, the negative electrode 1 was manufactured as follows.
A petroleum pitch appropriately selected from the range of H / C atomic ratio of 0.6 to 0.8 was crushed and subjected to oxidation treatment in an air stream to obtain a carbon precursor. At this time, the quinoline insoluble content (JIS centrifugal method: K2425-1983) of the carbon precursor was 80%, and the oxygen content rate (by organic element analysis method) was 15.
It was 4% by weight.

This carbon precursor was heated at 1000 ° C. in a nitrogen stream.
A soft graphitized carbon material was obtained by heating to a temperature of 10 ° C. and then pulverized to obtain a carbon material powder having an average particle size of 10 μm. Regarding the non-graphitizable carbon material obtained at this time, X
As a result of line diffraction measurement, the spacing between (002) planes was 3.
It was 76Å and the true specific gravity was 1.58.

90 parts by weight of the carbon material powder was mixed with 10 parts by weight of polyvinylidene fluoride as a binder to prepare a negative electrode mixture, and the negative electrode mixture was mixed with the solvent N-methyl-.
A negative electrode slurry was prepared by dispersing it in 2-pyrrolidone to form a slurry.

The negative electrode slurry thus obtained was evenly applied on both sides of a strip-shaped copper foil having a thickness of 10 μm to be a negative electrode current collector, dried and then compression-molded by a roll press machine to form a strip-shaped strip. A negative electrode 1 was produced.

Next, the positive electrode 2 was produced as follows. Lithium carbonate and cobalt carbonate were mixed in a ratio of 0.5 mol: 1 mol, and the mixture was baked in air at 900 ° C. for 5 hours to obtain LiCoO ₂ . LiC thus obtained
91 parts by weight of oO ₂ was mixed with 6 parts by weight of graphite as a conductive material and 3 parts by weight of polyvinylidene fluoride as a binder to prepare a positive electrode mixture, and the positive electrode mixture was dispersed in a solvent N-methyl-2-pyrrolidone. To prepare a positive electrode slurry.

Then, the positive electrode slurry thus obtained was uniformly applied to both sides of a strip-shaped aluminum foil having a thickness of 20 μm to be a positive electrode current collector, dried and then compression-molded by a roll press machine to form a strip. A positive electrode 2 was produced.

Then, as shown in FIG. 1, the strip-shaped negative electrode 1,
When the strip-shaped positive electrode 2 and the separator 3 made of a microporous polypropylene film are used as spiral electrode elements, the length and width are adjusted in advance so that they can be properly accommodated in the battery can 5 having a diameter of 20 mm and a height of 50 mm. Then, a spiral electrode was manufactured.

The spiral electrode thus manufactured was housed in a nickel-plated iron battery can 5, and insulating plates 4 were arranged on both upper and lower surfaces of the housed spiral electrode. Then, from the positive electrode current collector 10 to the aluminum positive electrode lead 12
From the negative electrode current collector 9 to the nickel negative electrode lead 1
1 was drawn out and welded to the battery can 5.

Then, LiPF ₆ was dissolved in a mixed solvent of 50% by volume of PC and 50% by volume of MEC at a ratio of 1 mol / l to prepare an electrolytic solution, and this electrolytic solution was injected into the battery can 5. Then, the battery can 5 is caulked via the insulating gasket 6 coated with asphalt to fix the battery lid 7, and to manufacture a cylindrical non-aqueous electrolyte battery (Example battery 1) having a diameter of 20 mm and a height of 50 mm. did.

Comparative Example 1 LiP was added to a mixed solvent of 50% by volume of PC and 50% by volume of DEC.
A cylindrical non-aqueous electrolyte battery (Comparative Example Battery 1) was produced in the same manner as in Example 1 except that an electrolytic solution prepared by dissolving F ₆ at a ratio of 1 mol / l was used.

Comparative Example 2 LiP was added to a mixed solvent of 50% by volume of PC and 50% by volume of DMC.
A cylindrical non-aqueous electrolyte battery (Comparative Example Battery 2) was produced in the same manner as in Example 1 except that an electrolytic solution prepared by dissolving F ₆ at a rate of 1 mol / l was used.

The example battery 1, the comparative battery 1, and the comparative battery 2 manufactured as described above were float-charged at 4.1 V under the condition of a temperature of 60 ° C., and the relationship between the float time and the battery internal pressure was examined. . The result is shown in FIG.

As can be seen from FIG. 2, the internal pressure of the comparative example battery 2 is larger than that of the example battery 1 and the comparative example battery 1. Therefore, from this, P as a non-aqueous solvent
It was found that the use of a mixed solvent of C and DMC is unsuitable for use in a high temperature environment.

Next, for each battery, the maximum charging voltage is 4.
Charged for 2.5h at 1V and charging current 1A, 6.2o
A charge / discharge cycle in which discharging was performed to a final voltage of 2.75 V with a constant resistance of hm was repeated, and changes in energy density with the charge / discharge cycle were examined. The result is shown in FIG.

As can be seen from FIG. 3, the battery of Example 1 and the battery of Comparative Example 2 show a good cycle characteristic with a small decrease in energy density with charge / discharge cycles, while the battery of Comparative Example 1 has a good cycle characteristic. Cycle deterioration is large. That is, the non-aqueous electrolyte secondary battery using the mixed solvent of PC and DEC has insufficient cycle characteristics.

Next, each battery was discharged at a constant resistance of 6.2 ohm in a temperature -20 ° C. environment, and the relationship between discharge time and voltage was examined. The result is shown in FIG. From FIG.
It can be seen that the battery of Example 1 has a smaller voltage drop during discharge than other batteries and exhibits excellent characteristics even in a low temperature environment.

From these results, a mixed solvent of PC and MEC is suitable as the non-aqueous solvent of the non-aqueous electrolyte secondary battery, and by using the mixed solvent of PC and MEC, the mixed solvent of PC and MEC has a high temperature in a wide temperature range. It was found that energy density and long life could be obtained.

For reference, the conductivity of the electrolytic solution used in each battery at various temperatures is shown in Table 1, and the frozen state is shown in Table 2.

[0056]

[Table 1]

[0057]

[Table 2]

From Table 2, the electrolytic solution using PC and MEC and the electrolytic solution using PC and DEC do not freeze even when kept at −50 ° C., while maintaining a liquid state.
Electrolytes using PC and DMC freeze at -30 ° C. On the other hand, looking at the electrical conductivity shown in Table 1, the electrolytic solution using PC and DEC has a lower electrical conductivity than the other electrolytic solutions.

That is, it is understood that the electrolytic solution using PC and MEC is excellent in both low temperature characteristics and conductivity.

Example 2 A spiral electrode having the same structure as in Example 1 was prepared except that the size was set so that the battery can properly fit in a battery can having a diameter of 18 mm and a height of 65 mm. The produced spiral electrode was housed in a nickel-plated iron battery can, and insulating plates were arranged on both upper and lower surfaces of the housed spiral electrode. Then, the aluminum positive electrode lead was led out from the positive electrode collector and the nickel lead was led out from the negative electrode collector and welded to the battery can.

Then, an electrolytic solution was prepared by dissolving LiPF ₆ at various concentrations in a mixed solvent of 30% by volume of PC and 70% by volume of MEC, and this electrolytic solution was injected into a battery can.
The battery lid was fixed by caulking the battery can through an insulating gasket coated with asphalt, and a cylindrical nonaqueous electrolyte battery having a diameter of 18 mm and a height of 50 mm was produced. In addition,
The concentration of LiPF ₆ dissolved in the mixed solvent was 0.5,
0.6, 0.8, 1.0, 1.2, 1.4, 1.6,
1.8 and 2.0 mol / l.

Example 3 LiP was added to a mixed solvent of 40% by volume of PC and 60% by volume of MEC.
A cylindrical non-aqueous electrolyte secondary battery (Example battery 3) was produced in the same manner as in Example 2 except that an electrolytic solution prepared by dissolving F ₆ at various concentrations was used.

Example 4 LiP was added to a mixed solvent of 50% by volume of PC and 50% by volume of MEC.
A cylindrical non-aqueous electrolyte secondary battery (Example battery 4) was produced in the same manner as in Example 2 except that an electrolytic solution prepared by dissolving F ₆ at various concentrations was used.

Example 5 LiP was added to a mixed solvent of 60% by volume of PC and 40% by volume of MEC.
A cylindrical non-aqueous electrolyte secondary battery (Example battery 5) was produced in the same manner as in Example 2 except that an electrolytic solution prepared by dissolving F ₆ at various concentrations was used.

Example 6 LiP was added to a mixed solvent of 70% by volume of PC and 30% by volume of MEC.
A cylindrical non-aqueous electrolyte secondary battery (Example battery 6) was produced in the same manner as in Example 2 except that an electrolytic solution prepared by dissolving F ₆ at various concentrations was used.

Comparative Example 3 LiP was added to a mixed solvent of 50% by volume of PC and 50% by volume of DEC.
A cylindrical non-aqueous electrolyte battery (Comparative Example Battery 3) was produced in the same manner as in Example 2 except that an electrolytic solution prepared by dissolving F ₆ at a ratio of 1 mol / l was used.

With respect to the battery manufactured as described above, the internal pressure of the battery after being left in a charged state at a temperature of 90 ° C. for 40 hours, the discharge capacity ratio and the conductivity at a temperature of -20 ° C., and the charge / discharge efficiency were measured. did. The charge / discharge efficiency is 1 charge current.
A, constant voltage charging was performed under the conditions of voltage 4.2V for 2.5 hours, and then discharge was repeated under conditions of discharge load 6.2Ω and cutoff voltage 2.75V. It was measured by determining the ratio of the capacity at the 100th cycle to the capacity (2nd cycle capacity / 100th cycle capacity).

Regarding the discharge capacity ratio, after the initial charging, the charging / discharging cycle was repeated 3 times under the above-mentioned conditions, and after the charging for the 4th cycle, the current was 700 mA under the environment of temperature -20 ° C. It was determined by measuring the discharge capacity when constant current discharge was performed. The internal pressure of the battery is measured after the initial charging, the charging / discharging cycle under the above-mentioned conditions for 3 cycles, and the charging for the 4th cycle. did.

FIG. 5 shows the relationship between the electrolyte concentration in the electrolytic solution and the discharge capacity ratio under the environment of temperature −20 ° C. and the concentration of the electrolyte in the electrolytic solution and the internal pressure of the battery after being left in the charged state under the environment of temperature 90 ° C. The relationship is shown in FIG. FIG. 7 shows the relationship between the MEC mixing rate and the conductivity of the electrolytic solution (in the figure, the concentration shown in mol / l is Li
PF ₆ concentration. 8) and 9 show the relationship between the electrolyte concentration in the electrolytic solution and the electrical conductivity. Note that FIG. 8 shows the case where the conductivity was measured under the temperature of 22 ° C. environment, and FIG. 9 shows the case where the conductivity was measured under the temperature −20 ° C. environment. Further, FIG. 10 shows the relationship between the electrolyte concentration in the electrolytic solution and the charge / discharge efficiency.

First, as can be seen from FIG. 5, the batteries of Examples 2 to 6 using the mixed solvent of MEC and PC as the non-aqueous solvent of the electrolytic solution were all DEC as the non-aqueous solvent.
Temperature as compared to Comparative Example Battery 3 using a mixed solvent of PC and PC
The discharge capacity ratio under the environment of 20 ° C is large and the low temperature characteristics are excellent.

Further, as shown in FIG. 6, in the case of Example battery 2 to Example battery 6, the internal pressure of the battery stops increasing to the same extent as in Comparative battery 3 which is said to have relatively high temperature characteristics. Therefore, it can be seen that the high temperature characteristics are sufficiently practical.

This is in agreement with the tendency shown by FIGS. 2 and 4 described above, and from this also, ME
It is supported that the use of the mixed solvent of C and PC as the non-aqueous solvent of the electrolytic solution is effective in obtaining a battery showing good characteristics in a wide temperature range.

Next, referring to FIG. 7, ME was used as the non-aqueous solvent.
In an electrolytic solution using a mixed solvent of C and PC, the conductivity changes depending on the MEC mixing ratio, increases on the low mixing ratio side of MEC as the MEC mixing ratio increases, and conversely increases the MEC mixing ratio. It can be seen that on the rate side, it decreases as the MEC mixing rate increases. In other words, in an electrolyte solution that uses a mixed solvent of MEC and PC as a non-aqueous solvent, the conductivity of ME increases.
There is a proper mixing ratio range of C, and the MEC mixing ratio is 30 to 70.
It can be seen that by setting the content by weight%, a practical conductivity can be obtained as the electrolytic solution of the battery.

Further, referring to FIGS. 8 and 9, in the electrolytic solution using the mixed solvent of MEC and PC as the non-aqueous solvent, the conductivity also changes depending on the concentration of the electrolyte, and on the low concentration side of the electrolyte, the conductivity of It can be seen that it increases with an increase, and conversely, it decreases with an increase in the electrolyte concentration on the high concentration side of the electrolyte.

Further, as can be seen from FIG. 10,
In a battery using a mixed solvent of MEC and PC as the non-aqueous solvent of the electrolytic solution, the charging / discharging efficiency also changes depending on the dissolved concentration of the electrolyte dissolved in the electrolytic solution, and the electrolyte concentration on the low concentration side of the electrolyte changes It can be seen that it increases with an increase, and conversely, it decreases with an increase in the electrolyte concentration on the high concentration side of the electrolyte.

That is, in the battery using the mixed solvent of MEC and PC as the non-aqueous solvent of the electrolytic solution, there is an appropriate concentration range of the electrolyte in which the electric conductivity and the charging / discharging efficiency are high, and the electrolyte concentration is 0.6 to 1.8 mol. It can be seen that by setting it as / l, practical conductivity and charge / discharge efficiency can be obtained.

The specific embodiments to which the present invention is applied have been described above, but the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention. . In addition, as a solvent to be mixed in the above examples, instead of propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 2
-Methyltetrahydrofuran, 1,3-dioxolane,
Similar effects were obtained when 4-methyl 1,3 dioxolane, sulfolane, methyl sulfolane, acetonitrile, propionitrile, dimethyl sulfoxide, etc. were used.

[0078]

As is apparent from the above description, the non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode made of a lithium composite oxide, a negative electrode made of a carbonaceous material, and an electrolyte in a non-aqueous solvent. Since the mixed solvent of MEC and PC is used as the non-aqueous solvent used for the non-aqueous electrolyte secondary battery having the dissolved electrolyte, the battery internal pressure is almost constant even when used in a high temperature environment. It does not rise and the electrolyte does not solidify even when used in a low temperature environment, and high energy density, long life, and high reliability can be obtained over a wide temperature range. Therefore, it can be said that it can be used even under the condition of, for example, a vehicle in the summer and is extremely practical.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing an example of a non-aqueous electrolyte secondary battery to which the present invention is applied.

FIG. 2 is a characteristic diagram showing a relationship between a float time and a battery internal pressure of a non-aqueous electrolyte secondary battery.

FIG. 3 is a characteristic diagram showing the relationship between the number of charge / discharge cycles and energy density of a non-aqueous electrolyte secondary battery.

FIG. 4 is a characteristic diagram showing a relationship between a discharge time and a voltage of a non-aqueous electrolyte secondary battery under a temperature −20 ° C. environment.

FIG. 5: LiP in electrolyte at a temperature of -20 ° C.
It is a characteristic diagram showing the relationship of F ₆ concentration and the discharge capacity ratio.

FIG. 6 shows a LiPF ₆ concentration in the electrolytic solution and a temperature of 9 in a charged state.
It is a characteristic view which shows the relationship of the battery internal pressure when left to stand in a 0 degreeC environment.

FIG. 7 is a characteristic diagram showing the relationship between the MEC mixing ratio of the electrolytic solution and the conductivity.

FIG. 8: LiPF in electrolyte at a temperature of 22 ° C.
FIG. ₆ is a characteristic diagram showing the relationship between ₆ concentration and conductivity.

FIG. 9: LiP in electrolyte at a temperature of -20 ° C.
F ₆ is a characteristic diagram showing the relationship between the concentration and electric conductivity.

FIG. 10 is a characteristic diagram showing the relationship between the LiPF ₆ concentration in the electrolytic solution and the charge / discharge efficiency.

[Explanation of symbols]

 1 ... Negative electrode 2 ... Positive electrode 3 ... Separator 4 ... Insulating plate 5 ... Battery can 6 ... Insulating gasket 7 ... Battery lid 9 ... Negative electrode current collector 10 ...・ Positive electrode current collector 11 ... Negative electrode lead 12 ... Positive electrode lead

Claims (3)

[Claims] 1. A negative electrode made of a carbonaceous material capable of being doped and dedoped with lithium ions, and Li _X MO ₂ (provided that M
Represents at least one of cobalt, nickel and manganese. ) And a non-aqueous electrolyte secondary battery comprising an electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent, wherein the non-aqueous solvent is a mixed solvent of propylene carbonate and methyl ethyl carbonate. Characteristic non-aqueous electrolyte secondary battery. 2. The non-aqueous electrolyte solution according to claim 1, wherein the non-aqueous solvent is a mixed solvent in which propylene carbonate and methyl ethyl carbonate are mixed in a volume ratio of 7: 3 to 3: 7. Secondary battery. 3. The non-aqueous electrolyte secondary battery according to claim 2, wherein LiPF ₆ as an electrolyte is dissolved in the non-aqueous solvent at a concentration of 0.6 to 1.8 mol / l.