KR20040092425A - Non-aqueous electrolytic solutions for lithium battery and lithium ion secondary battery - Google Patents

Abstract

PURPOSE: A nonaqueous electrolyte solution for a lithium battery and a lithium ion secondary battery using the nonaqueous electrolyte solution are provided, to prevent the reductive cleavage of an electrolyte solution at an anode, to reduce the interface resistance between an anode or a cathode and the corresponding electrolyte solution and to inhibit the deterioration of lithium ion conductivity due to thickening. CONSTITUTION: The nonaqueous electrolyte solution comprises a lithium salt; a nonaqueous solvent; 0.1-10 wt% of a silyl ester compound; and 0.01 wt% or more of a tetrafluoroborate salt. Preferably the ratio of the tetrafluoroborate salt to the silyl ester compound (the amount of the tetrafluoroborate salt / the amount of the silyl ester compound) is 0.05 or more. Preferably the silyl ester compound is trimethylsilyl phosphate and the tetrafluoroborate salt is lithium tetrafluoroborate.

Description

Non-aqueous electrolyte and lithium ion secondary battery for lithium battery {NON-AQUEOUS ELECTROLYTIC SOLUTIONS FOR LITHIUM BATTERY AND LITHIUM ION SECONDARY BATTERY}

The present invention relates to a nonaqueous electrolyte and a lithium ion secondary battery for a lithium battery.

Lithium batteries, particularly lithium ion secondary batteries, have high voltage and energy density, are easy to miniaturize, and are widely used as power sources for various consumer electronic devices, especially portable electronic devices such as mobile phones and mobiles, and demand thereof is increasing.

Lithium ion secondary batteries mainly include a negative electrode made of carbon material such as graphite having excellent lithium occlusion and release properties, a positive electrode made of a composite compound of lithium and a transition metal, and a nonaqueous electrolyte. As the non-aqueous electrolyte, for example, LiPF ₆ , LiPF ₄ , LiN ( SO ₂ CF _{3) 2,} it is widely used by the addition of a lithium salt of _{LiN (SO 2 C 2 F 5} ) 2 or the like. Moreover, in order to provide flame retardance to a nonaqueous electrolyte solution, using the silyl ester compound as one component of a solvent is also proposed (for example, refer patent document 1). However, a common nonaqueous electrolyte has a drawback that the electrolyte is easily decomposed at the surface of the electrically active electrode, and the decomposition product increases the internal resistance of the battery. This drawback causes a problem that the charge / discharge characteristics of the battery are lowered and the battery life is shortened.

The decomposition of the nonaqueous electrolyte at the electrode surface includes, for example, reduction decomposition at the negative electrode surface. For such a problem, vinylene carbonate (see, for example, Patent Document 2), vinylethylene carbonate (see, for example, Patent Document 3), ethylene sulfite (see, for example, Nonpatent Document 1) in the nonaqueous electrolyte solution. ), Sultones (see, for example, Patent Document 4), and the like are added. These additives are intended to prevent the decomposition of the electrolyte solution by forming an ion conductive protective film on the negative electrode surface and to improve the charge and discharge characteristics of the battery.

Moreover, it is proposed to add a phosphate compound to a nonaqueous electrolyte (for example, refer patent document 5). According to patent document 5, in a lithium ion secondary battery, the phosphate compound contained in the nonaqueous electrolyte reduces the irreversible capacity in a negative electrode in initial charge and discharge. Therefore, by reducing the irreversible capacity, battery cycle characteristics from room temperature to low temperature are improved. Although many phosphate esters are illustrated as a phosphate compound, the effect is not specifically indicated about the phosphate esters other than dibenzyl phosphate.

Moreover, it is proposed to add a silyl ester compound to a nonaqueous electrolyte (for example, refer patent document 6). According to patent document 6, the silyl ester compound is a lithium ion secondary battery in which the reduction of the nonaqueous electrolyte is carried out in the negative electrode. To prevent. Further, Patent Document 6 suggests that lithium tetrafluoroborate may be used as an electrolyte in a nonaqueous electrolyte containing a silyl ester compound. However, the content of lithium tetrafluoroborate in the nonaqueous electrolyte is an amount for allowing lithium tetrafluoroborate to act as an electrolyte. Patent Literature 6 does not suggest the peculiar effect obtained by using lithium tetrafluoroborate and a silyl ester compound in combination and by using a small amount of lithium tetrafluoroborate in a small amount that does not sufficiently function as an electrolyte.

As mentioned above, it is known that a silyl ester compound is useful for the purpose of improving the charge / discharge characteristic of a lithium ion secondary battery.

However, in this state, it is urgently required to further extend the use time corresponding to one charge of the mobile device. To this end, it is necessary to further increase the energy density of the battery by further increasing the charge density of the active material in the lithium ion secondary battery as a power source. However, when the packing density of the active material is increased, the amount of electrolyte in the battery is relatively decreased, and the ion conductivity between the negative electrode and the positive electrode is lowered. In addition, since the surface area per weight of the electrode active material is small, the interface resistance between the electrode and the electrolyte is increased, and the battery internal resistance is increased. As a result, the load characteristic, low temperature characteristic, etc. of a battery fall. In addition, since the influence of the electrolytic solution electrolysis is stronger as a decrease in the charge / discharge characteristics of the battery, the charge / discharge cycle characteristics, the high temperature storage characteristics, and the like tend to decrease.

Therefore, in a lithium ion secondary battery having a high energy density, a nonaqueous electrolyte having excellent load characteristics, low temperature characteristics, charge / discharge cycle characteristics, high temperature storage characteristics, and the like of the battery is required.

[Patent Document 1] Publication No. 3-236169

[Patent Document 2] Japanese Patent Application Laid-Open No. 5-13088

[Patent Document 3] Japanese Patent Application Laid-Open No. 2000-40526

[Patent Document 4] Publication No. 11-162511

[Patent Document 5] Japanese Patent Application Laid-Open No. 2000-331710

[Patent Document 6] Japanese Patent Application Laid-Open No. 2001-57237

[Non-Patent Document 1] Journal of Electrochemical Society, 146 (2), 470-472 (1999)

In the course of the present invention, a silyl ester compound, particularly a phosphate silyl ester compound, has a reduction prevention effect on a nonaqueous electrolyte in a negative electrode, but a lithium ion secondary battery having a high energy density. It discovered that the effect was not fully exhibited.

That is, since the silyl ester compound decreases the mobility of lithium ions in the nonaqueous electrolyte and, moreover, the lithium ion dielectric property due to its thickening (increasing the viscosity of the liquid), the battery internal resistance is increased in lithium ion secondary batteries with high energy density. It turns out to cause an increase. It has also been found that the non-aqueous electrolyte containing the silyl ester compound may release a large amount of decomposition gas when stored at a high temperature of 60 ° C. or higher, resulting in defects such as an increase in the thickness of the battery.

As a result of further research on the basis of the above knowledge, the present inventors have resolved the thickening of the nonaqueous electrolyte by the silyl ester compound to obtain a nonaqueous electrolyte for lithium batteries having a novel composition that can sufficiently exhibit the effect by the silyl ester compound. It succeeded, and completed this invention.

An object of the present invention is to solve the drawbacks of the silyl ester compound and to exhibit the action of the silyl ester compound at a higher level in the non-aqueous electrolyte for lithium batteries, thereby providing charge and discharge cycle characteristics, load characteristics, and low temperature characteristics over a wide temperature range. It is to provide a very excellent non-aqueous electrolyte for lithium batteries.

In addition, by using the non-aqueous electrolyte solution for lithium batteries described above, the charge and discharge cycle characteristics of the negative electrode active material are high, but the charge / discharge cycle characteristics are not deteriorated, and the lithium ion secondary battery has excellent load characteristics, low temperature characteristics, and high temperature storage characteristics. Can be provided.

1 is a graph showing the relationship between the content of the silyl ester compound and the viscosity in the nonaqueous electrolyte of the present invention.

2 is a graph showing charge and discharge cycle test results at 25 ° C. of the coin-type lithium ion secondary batteries of Examples 50 to 53 and Comparative Examples 16 to 18. FIG.

3 is a graph showing charge and discharge cycle test results at 25 ° C. of the coin-type lithium ion secondary batteries of Examples 54 and 55 and Comparative Examples 19 to 21;

The nonaqueous electrolyte solution for lithium batteries of the present invention is a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent, and further contains 0.1 to 10% by weight of the total amount of the nonaqueous electrolyte solution and 0.01% by weight or more of the total amount of the nonaqueous electrolyte solution to the silyl ester compound. It is characterized in that it is added so as to.

Moreover, the nonaqueous electrolyte solution for lithium batteries of this invention, Preferably, the weight ratio (content of tetrafluoroborate / content of a silyl ester compound) of the said silyl ester compound and tetrafluoroborate is 0.05 or more, It is characterized by the above-mentioned.

Moreover, the nonaqueous electrolyte solution for lithium batteries of the present invention is preferably characterized in that the silyl ester compound is trimethylsilyl ester of phosphate.

The nonaqueous electrolyte solution for lithium batteries of the present invention is preferably characterized in that the tetrafluoroborate is lithium tetrafluoroborate.

Moreover, the lithium ion secondary battery of this invention is comprised including the negative electrode, the positive electrode, and electrolyte solution which can occlude and discharge | release lithium, It is characterized by the electrolyte solution any one of said non-aqueous electrolyte solution.

The nonaqueous electrolyte solution for lithium batteries of the present invention is a nonaqueous electrolyte solution containing a lithium salt (except lithium tetrafluoroborate) and a nonaqueous solvent, and further contains a silyl ester compound and a tetrafluoroborate.

As the lithium salt, those commonly used in the art may be used, for example, LiPF ₆ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{4 (2k + 1)} (where k is 1 LiPF _n (C _k F _{(2k + 1)} ) _(6-n) (wherein k represents the same as above, n represents an integer of 0 to 5), LiC (SO ₂ R ¹ ) (SO ₂ R ² ) (SO ₂ R ³ ), LiN (SO ₂ OR ⁴ ) (SO ₂ OR ⁵ ), LiN (SO ₂ R ⁶ ) (SO ₂ R ⁷ ) (R ¹ ~ R <^7> is the same or different, and represents a C1-C8 perfluoroalkyl group), etc. are mentioned. Among these, LiPF ₆ , LiN (SO ₂ R ⁶ ) (SO ₂ R ⁷ ) (wherein R ⁶ and R ⁷ are the same as above) are preferable. Among them, lithium hexafluorophosphate is most preferred. A lithium salt may be used individually by 1 type, and may use 2 or more types together.

In the present invention, when a specific amount of the silyl ester compound and the tetrafluoroborate is added to a nonaqueous electrolyte for lithium batteries containing lithium salts, in particular lithium hexafluorophosphate as an electrolyte and containing a nonaqueous solvent, the interfacial resistance between the negative electrode, the positive electrode and the electrolyte solution The effect of keeping this low is shown at a high level, and the thickening of the electrolyte solution by the silyl ester compound is prevented. Therefore, the reduction decomposition of electrolyte solution in a negative electrode can be prevented, and the action of a silyl ester compound can fully be exhibited.

In addition, when only tetrafluoroborate is added, the interfacial resistance between the negative electrode and the electrolyte is increased, and the lithium ion chargeability to the negative electrode is deteriorated, so that the charge / discharge cycle characteristics at low temperatures are particularly low. Moreover, there exists a fault that the self-discharge property of a battery becomes large and the battery capacity after high temperature storage falls. However, when a specific amount of silyl ester compound is added together with tetrafluoroborate, the defects of both are solved and the advantages of both are synergistically shown.

That is, in the nonaqueous electrolyte solution for lithium batteries of the present invention, the unexpected effect that the thickening of the electrolyte solution by the silyl ester compound is solved by the tetrafluoroborate, and the effect that the defect of the tetrafluoroborate is solved by the silyl ester compound and By adding a specific amount of the silyl ester compound and the tetrafluoroborate, respectively, the effect of keeping the interfacial resistance between the negative electrode, the positive electrode and the electrolyte solution low is compensated without offsetting each other. The use of such a non-aqueous electrolyte for lithium batteries is excellent in charge and discharge cycle characteristics, load characteristics and low temperature characteristics, and lithium, which is less deteriorated in each of the above characteristics, particularly in the charge / discharge cycle characteristics, even if the charge density of the negative electrode active material is increased to increase the energy density. An ion secondary battery can be obtained.

The reason why nonaqueous electrolyte thickening is prevented by using a silyl ester compound and tetrafluoroborate in combination is unclear, but it is estimated as follows.

Since the silyl ester compound is a compound with strong donor properties, it forms a large molecular weight molecule by coordinating a large number with lithium ions, reduces the mobility of lithium ions in the electrolyte solution, and simultaneously increases the viscosity of the lithium ions in the electrolyte solution. Further reduces mobility.

On the other hand, if tetrafluoroborate is present, the silyl ester compound preferentially coordinates with the boron co-orbit in the tetrafluoroborate to form a complex and does not coordinate with lithium ions. In addition, since the silyl ester compound does not coordinate with boron in a large number and a molecule having a large molecular weight is not formed, a viscosity increase does not occur. Alternatively, since the silyl group of the silyl ester compound has high reactivity with fluorine, the silyl group of the silyl ester compound reacts with the fluorine of the tetrafluoroborate and is released, and the reactant of the silyl ester compound and the tetrafluoroborate is converted into the lithium ion of the silyl ester compound. It is assumed that the coordination of 없어 disappears and the viscosity does not occur. As a result, the mobility of lithium ions in the electrolyte solution is prevented from being disturbed by the silyl ester compound.

The reason why the increase in the interfacial resistance between the negative electrode and the electrolyte by the tetrafluoroborate, the increase in the self-discharge of the battery, and the decrease in capacity after high temperature storage are prevented by complexing the silyl ester compound and the fluoroborate or forming a condensate or a reactant It is guessed that the reducing property in the negative electrode of a fluoroborate falls.

The nonaqueous electrolyte of the present invention also includes one or two or more selected from coordination complexes, condensates and reactants of the silyl ester compound and tetrafluoroborate. Furthermore, all of the silyl ester compounds or tetrafluoroborate are changed to coordination complexes, condensates or reactants. Such coordination complexes, condensates, reactants, etc. do not impair the excellent effect of the nonaqueous electrolyte solution of the present invention, but it is speculated that excellent effects of the nonaqueous electrolyte solution of the present invention can be obtained by forming coordination complexes, condensates, reactants, and the like. It is as described above. Therefore, the nonaqueous electrolyte of the present invention is not only containing 1) a silyl ester compound and tetrafluoroborate together with lithium salts and non-aqueous solvents as electrolytes, but also 2) silyl ester compounds and coordination of silyl ester compounds with tetrafluoroborate. 3) When it contains at least 1 type, such as a complex, a condensate, and a reactant, 3) When it contains at least 1 sort (s) of coordination complexes, condensates, reactants, etc. of a tetrafluoroborate and a tetrafluoroborate and a silyl ester compound, and 4) It also includes the case where at least 1 sort (s) of coordination complexes, condensates, and reactants of a silyl ester compound and tetrafluoroborate are included.

Lithium salt is used as the electrolyte of the nonaqueous electrolyte. The content of the lithium salt is not particularly limited, and may be appropriately selected from a wide range depending on various conditions such as the type of the non-aqueous solvent, the type and content of the silyl ester compound and the tetrafluoroborate, and the electrode type when forming the battery. It is preferably included in the non-aqueous electrolyte at a concentration of 0.5 to 2 mol / liter, preferably 0.7 to 1.6 mol / liter.

As a silyl ester compound, a well-known thing can be used, For example, phosphoric acid silyl ester, sulfuric acid silyl ester, sulfurous acid silyl ester, alkyl sulfonic acid silyl ester, phenyl sulfonic acid silyl ester, carbonate silyl ester, carbonic acid silyl ester, boric acid silyl ester , Aluminic acid silyl ester, titanic acid silyl ester, trimethylsilyl acetate ester, and the like.

Among them, phosphoric acid silyl ester, sulfuric acid silyl ester, alkyl sulfonic acid silyl ester, and phenyl sulfonic acid silyl ester are preferable because they are excellent in reducing the electrode / electrolyte interface resistance, electrochemical stability, and the like. Phosphoric acid silyl esters are most preferred because they are particularly excellent in the above-mentioned functions and are easy to handle in the preparation of electrolyte solutions.

Specific examples of the silyl phosphate include, for example, tris (trimethylsilyl) phosphate, di (trimethylsilyl) phosphate, mono (trimethylsilyl) phosphate, dimethyltrimethylsilyl phosphate, methylbis (trimethylsilyl) phosphate, and diethyltrimethyl phosphate. Silyl, ethyl bis (trimethylsilyl), dipropyltrimethylsilyl phosphate, propylbis (trimethylsilyl) phosphate, dibutyl trimethylsilyl phosphate, butyl bis (trimethylsilyl phosphate), dioctyl trimethylsilyl phosphate, octyl bis (trimethylsilyl phosphate) ), Diphenyl trimethylsilyl phosphate, phenyl bis (trimethylsilyl), di (trifluoroethyl) phosphate (trimethylsilyl), trifluoroethyl bis (trimethylsilyl) phosphate, and the trimethylsilyl group of the above-mentioned silyl ester Compounds substituted with ethylsilyl group, triphenylsilyl group, dimethylethylsilyl group and the like, and silyl phosphate esters are condensed, and the number of people intervenes with oxygen. Combining, and the like compounds having a so-called condensed phosphoric acid ester structure. Among them, dimethyltrimethylsilyl phosphate, diethyltrimethylsilyl phosphate, methylbis (trimethylsilyl) phosphate, ethylbis (trimethylsilyl) phosphate, mono (trimethylsilyl phosphate), di (trimethylsilyl) phosphate, tris (trimethylsilyl), etc. This is preferable and tris phosphate (trimethylsilyl) is especially preferable. A silyl ester compound can be used individually by 1 type, or can use 2 or more types together.

Although the content of a silyl ester compound can be suitably selected from a wide range according to various conditions, such as the kind of a silyl ester compound, content of a lithium salt, the kind and content of tetrafluoroborate and a non-aqueous solvent, and the electrode type in the case of a battery, Usually, 0.1 weight% or more of the total amount of non-aqueous electrolyte solution, Preferably it is 0.1-10 weight%, More preferably, it is 0.1-3 weight%, Especially preferably, it is 0.2-1 weight%. Within this range, the increase in interfacial resistance between the negative electrode / electrolyte caused by tetrafluoroborate can be sufficiently reduced, and the increase in electrolyte decomposition gas when stored at a high temperature above 60 ° C., which is remarkable when there are many silyl ester compounds, It rarely causes problems such as increase in battery thickness.

As a tetrafluoroborate, a well-known thing can be used, For example, lithium tetrafluoroborate, sodium tetrafluoroborate, potassium tetrafluoroborate, tetramethylammonium tetraborate, tetraethylammonium tetraborate, tetrabutyl borate tetrafluoroborate Ammonium, trimethyl ethyl ammonium tetraborate, an imidazolium salt tetraborate, etc. are mentioned. Especially, lithium tetrafluoroborate is especially preferable from the cost, the ease of handling, the thickening inhibitory effect by a silyl ester compound, etc. A tetrafluoroborate may be used individually by 1 type, and may use 2 or more types together.

The content of tetrafluoroborate can be appropriately selected depending on various conditions such as the type of tetrafluoroborate, the content of lithium salt, the type and content of the silyl ester compound and the non-aqueous solvent, and the electrode type when forming the battery. The amount is 0.01% by weight or more, preferably 0.1 to 15% by weight, more preferably 0.1 to 3% by weight, particularly preferably 0.2 to 2% by weight. Within this range, the thickening of the electrolyte solution by the silyl ester compound can be prevented, and the silyl ester increases in the interfacial resistance between the negative electrode / electrolyte solution by the tetrafluoroborate when the tetrafluoroborate is large, or increases in the self-discharge resistance or decreases in capacity at high temperatures. It can be suppressed by a compound and it is less likely to cause problems, such as an increase in electrolyte decomposition gas and the thickness of a battery, when it is stored at 60 degreeC or more high temperature.

In the nonaqueous electrolyte of the present invention, the content ratio (weight ratio, content of tetrafluoroborate / silyl ester compound) of the silyl ester compound and the tetrafluoroborate is 0.05 or more, preferably 0.1 or more, more preferably 0.5 or more. 10 or less, More preferably, it is 1 or more and 4 or less.

By setting it as the said range, the electrolyte solution thickening by a silyl ester compound is suppressed effectively, and the increase of the interface resistance between the negative electrode / electrolyte by tetrafluoroborate, the increase of the self-discharge property at the high temperature, and the capacity | capacitance fall further are reduced. At the same time, the effect of maintaining the low interfacial resistance between the negative electrode and the positive electrode and the electrolyte solution appears to be at a higher level, thereby improving the charge / discharge cycle characteristics from low temperature to high temperature, and in particular, the load characteristics and low temperature characteristics are particularly improved. The increase in interfacial resistance during storage is particularly suppressed, which further improves high temperature storage.

As a preferable aspect of the nonaqueous electrolyte of the present invention, the content of the silyl ester compound is 0.1% by weight or more, preferably 0.1 to 10% by weight, more preferably 0.1 to 3% by weight, still more preferably 0.2 of the total amount of the nonaqueous electrolyte. ~ 1% by weight, particularly preferably in the range of 0.2 to 0.5% by weight, and the content of tetrafluoroborate is 0.01% by weight or more, preferably 0.1 to 3% by weight of the total amount of the non-aqueous electrolyte, more preferably 0.2 to 2% by weight, particularly preferably 0.3 to 0.7% by weight, and the content ratio (weight ratio) of the silyl ester compound and the tetrafluoroborate is 0.1 or more, preferably 1 to 20, more preferably 2-20, More preferably, it is 0.5-10, Especially preferably, it is good to set it as 1-4.

The nonaqueous electrolyte of the present invention contains a nonaqueous solvent together with lithium salt, silyl ester compound and tetrafluoroborate. As a nonaqueous solvent, an aprotic organic solvent can be used. Among the aprotic organic solvents, esters are preferable in consideration of electrochemical redox stability, chemical stability and the like. As ester, a cyclic ester, a chain ester, etc. are mentioned, for example. As a specific example of cyclic ester, For example, cyclic carbonates, such as ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, and trifluoromethylethylene carbonate, cyclic butyrolactone, etc. Can be mentioned. Specific examples of the chain ester include, for example, chains such as dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl trifluoroethyl carbonate, ditrifluoroethyl carbonate, diethyl carbonate, dibutyl carbonate and methyl octyl carbonate. Linear carboxylic acid esters such as carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, pentafluoropropyl acetate, and methyl trifluoroacetate. A nonaqueous solvent may be used individually by 1 type, or may use 2 or more types together.

Among such non-aqueous solvents, it is preferable to use a cyclic ester and a chain ester together in consideration of the fact that the charge and discharge characteristics of the battery can be stably exhibited even at low temperatures and regardless of the magnitude of the load on the battery. . In addition to improving the load characteristics and low temperature characteristics of the battery, and taking into account the electrochemical stability of the nonaqueous electrolyte, the cyclic ester is used as the cyclic ester in the combination system of the cyclic ester and the chain ester, and the chain ester as the chain ester. Preference is given to using carbonates.

The mixing ratio of the cyclic ester and the chain ester is not particularly limited and may be appropriately selected from a wide range depending on the type and content of the lithium salt, the silyl ester compound and the tetrafluoroborate salt, and other conditions, but usually cyclic carbonate: chain carbonate ( Weight ratio) is 5: 95-80: 20, Preferably it is 10: 90-70: 30, More preferably, it is 15: 85-55: 45. By adopting such a ratio, the viscosity rise of the electrolyte can be suppressed, the dissociation of the electrolyte, and further, the ion conductivity of the electrolyte can be increased, and the charge and discharge of the battery can be improved. In addition, since the solubility of the electrolyte is further increased to obtain an electrolyte having high ion conductivity at room temperature to low temperature, the load characteristics of the battery from room temperature to low temperature can be improved.

From the viewpoint of preventing the ignition of the electrolyte solution and further improving the safety of the battery, it is preferable to use cyclic ester alone as the nonaqueous solvent or to make the content of the chain ester be 20% by weight or less of the total amount of the nonaqueous solvent. In this case, the cyclic ester is preferably ethylene carbonate, propylene carbonate, γ-butyrolactone, a mixture of two or more thereof. Moreover, as a chain ester, chain carbonate is preferable.

The non-aqueous electrolyte of the present invention may contain cyclic carbonates having a vinyl group in a range not losing the desirable properties. By adding cyclic carbonates having a vinyl group, the reduction decomposition reaction of the nonaqueous solvent in the negative electrode is further prevented, and the high temperature storage property of the battery is further improved.

As the cyclic carbonates having a vinyl group, known ones can be used. For example, vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, propylvinylene carbonate, phenylvinylene carbonate, dimethylvinylene carbonate, diethyl Vinylene carbonate, dipropylvinylene carbonate, diphenylvinylene carbonate, vinylethylene carbonate, 4,5-divinylethylene carbonate, and the like. Especially, vinyl ethylene carbonate, divinyl ethylene carbonate, vinylene carbonate, etc. are preferable, and vinylene carbonate is especially preferable. The cyclic carbonates having a vinyl group may be used alone or in combination of two or more. As a preferable combination at the time of using 2 or more types together, vinylene carbonate and vinyl ethylene carbonate, vinylene carbonate, divinyl ethylene carbonate, etc. are mentioned, for example. The content of the cyclic carbonates having a vinyl group is not particularly limited, and may be appropriately selected from a wide range depending on the type, content of lithium salt, silyl ester compound, tetrafluoroborate, and non-aqueous solvent, and the like. 0.1 to 10% by weight of the amount, preferably 0.5 to 5% by weight.

The non-aqueous electrolyte of the present invention may contain a solvent other than the solvent, an additive commonly used in the art, and the like within a range not to lose its desirable properties. As such a solvent and an additive, For example, ethers, such as dimethoxyethane and tetrahydrofuran, amides, such as N-methylpyrrolidone and dimethylformamide, methyl-N, N- dimethyl carbamate, N Carbamates such as methyloxazolidinone, ureas such as N, N-dimethylimidazolidinone, 1,3-propanesultone, 1,4-butanesultone, ethylene sulfide, dimethyl sulfuric acid, diethyl sulfuric acid and the like Sulfuric acid esters, boric acid esters such as triethyl borate, tributyl borate, phosphoric acid esters such as trimethyl phosphate, trioctyl phosphate, benzene, toluene, xylene, diphenyl ether, fluorodiphenyl ether, phenylcyclohexane, fluoro And aromatic compounds such as robenzene, fluorotoluene, chlorobenzene, chlorotoluene, biphenyl, fluorobiphenyl, fluoroanisole, and fluorinated ethers such as trifluoroethylmethyl ether. These solvents and additives may be used individually by 1 type, or may use 2 or more types together.

The non-aqueous electrolyte of the present invention is a lithium salt, a silyl ester compound and a tetrafluoroborate, an appropriate amount of other solvents, an appropriate amount of additives, if necessary, and a non-aqueous solvent so that the total amount is 100% by weight, non-aqueous It can manufacture by dissolving each said component in a solvent.

The lithium ion secondary battery of this invention is obtained by using the nonaqueous electrolyte solution of this invention obtained in this way.

The lithium ion secondary battery of the present invention comprises a negative electrode capable of occluding and releasing lithium, a positive electrode, and a nonaqueous electrolyte solution of the present invention, except that the nonaqueous electrolyte solution of the present invention is included as an electrolyte solution, such as a known lithium ion secondary battery. Has a structure.

For example, a negative electrode current collector, a negative electrode capable of occluding and releasing lithium, a separator, a positive electrode, and a positive electrode current collector are stacked in this order so as to be in contact with the negative electrode can of the battery can, and then the battery can lid which is a positive electrode can through a gas blanket. The lithium ion secondary battery which has a structure which is equipped with and which impregnated the nonaqueous electrolyte solution of this invention is mentioned.

The negative electrode contains a negative electrode active material capable of occluding and releasing lithium. As such a negative electrode active material, well-known ones can be used. For example, lithium-containing alloys, silicon which can be alloyed with lithium, silicon alloys, tin, tin alloys, tin oxides which can occlude and release lithium, silicon oxides, And transition metal oxides capable of occluding and releasing lithium, transition metal nitrides capable of occluding and releasing lithium, and carbon materials capable of occluding and releasing lithium. Especially, the carbon material which can occlude and discharge | release lithium is preferable. As the carbon material, for example, fibrous, spherical (or particulate), potato, or flakes can be used. Specific examples of the carbon material include, for example, crystalline carbon such as carbon black, activated carbon, and graphite materials (for example, natural graphite, artificial graphite, graphitized mesocarbon microbeads, and graphitized mesophase beach carbon fibers). Amorphous carbon such as carbon, coke, mesocarbon microbead baked at 1500 ° C. or lower, and mesophase beach carbon fiber; and the like can be given. In addition, a carbon material containing boron, or a carbon material coated with a metal such as gold, platinum, silver, copper, or tin can be used. A negative electrode active material may be used individually by 1 type, and may use 2 types together.

The negative electrode is, for example, i) molded into a desired composition comprising a negative electrode active material, a binder, and a conductive aid such as carbon black, if necessary, the molded article is adhered to the negative electrode current collector, and if necessary, a press Or ii) adding a solvent to the composition comprising the negative electrode active material and the binder to form a negative electrode mixture slurry, applying the slurry to one side of the negative electrode current collector and drying, and performing a pressurization as necessary. Or iii) forming the negative electrode active material into a desired shape by roll molding, compression molding, or the like. In the method of i), what is commonly used in this field can be used as a binder, For example, fluororesins, such as polyvinylidene fluoride and polytetrafluoroethylene, celluloses, such as carboxymethylcellulose and cellulose, styrene Latexes, such as butadiene rubber, isoprene rubber, butadiene rubber, ethylene propylene rubber, natural rubber, etc. are mentioned. A well-known thing can also be used also as a negative electrode collector, For example, copper, nickel, stainless steel, etc. are mentioned. In the method of ii), the same thing as the method of i) can be used as a binder. As the solvent, those which are commercially available in the art can be used, for example, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, propylene carbonate, γ-butyrolactone, N-methyloxazolidinone, water, Alcohol etc. are mentioned. A solvent may be used individually by 1 type, and may use 2 or more types together as needed.

In the lithium ion secondary battery of the present invention, the negative electrode active material is a carbon material capable of occluding and releasing lithium, and the negative electrode mixture layer (negative electrode formed on the current collector) is formed by compression molding the negative electrode through the step (iii). The packing density of the negative electrode active material in the mixture layer of the active material, the binder, and the conductive aid is preferably 1.5 g / ml or more, preferably 1.55 g / ml or more, and more preferably 1.65 g / ml or more. Since the nonaqueous electrolyte solution of the present invention is used, a lithium ion secondary battery having a high energy density and excellent charge / discharge cycle characteristics and high temperature storage characteristics can be obtained by increasing the charge density of the negative electrode.

In addition, when the charge density of the negative electrode active material is high, the energy density of the battery can be increased. However, since the porosity in the negative electrode mixture layer contributing to ion conductivity decreases, the charge and discharge of lithium ions to the negative electrode decreases. In addition, since the surface area per weight of the negative electrode active material is small, the interface resistance between the negative electrode and the debris becomes large. As a result, when the conventional nonaqueous electrolyte is used, deterioration of charge and discharge cycle characteristics and high temperature storage characteristics tends to occur. However, when the nonaqueous electrolyte of the present invention is used, the interfacial resistance between the negative electrode and the electrolyte is kept low, so that good charge and discharge cycle characteristics and high temperature storage characteristics can be obtained.

The positive electrode contains a positive electrode active material capable of occluding and releasing lithium. As the positive electrode active material, those commercially available in the art may be used. For example, oxides or sulfides of transition metals such as FeS ₂ , MoS ₂ , TiS ₂ , MnO ₂ , V ₂ O ₅ , LiCoO ₂ , LiMnO ₂ , LiMn Composite oxides containing lithium and transition metals such as ₂ O ₄ , LiNiO ₂ , LiNi _x Co _(1-x) O ₂ , LiNi _x Co _y Mn _(1-xy) O ₂ , polyaniline, polythiophene, polypyrrole, Conductive polymer materials such as polyacetylene, polyacene, dimercaptothiadiazole / polyaniline composite, carbon materials such as fluorocarbon, activated carbon, and the like. Among these, the composite oxide containing lithium and a transition metal is preferable. A positive electrode active material may be used individually by 1 type, and may use 2 or more types together.

The positive electrode may contain a conductive assistant together with the positive electrode active material. As the conductive aid, those commercially available in the art can be used, and examples thereof include carbon black, amorphous whiskers, graphite and the like.

The positive electrode can be produced in the same manner as in the manufacturing method of the negative electrode described above, except that a positive electrode active material or a positive electrode active material and a conductive assistant are used instead of the negative electrode active material, and a positive electrode current collector is used instead of the negative electrode current collector. As the positive electrode current collector, those commercially available in the art can be used. For example, a passivation film is formed on the surface by anodization in an electrolyte solution such as Al, Ti, Zr, Hf, Nb, Ta, and alloys containing these. The metal to form etc. are mentioned.

In the lithium ion secondary battery of the present invention, one or two or more selected from lithium phosphate, lithium sulfate, and lithium fluoride in or on the surface of the positive electrode mixture layer formed on the positive electrode surface is 0.01 to 2% by weight based on the weight of the positive electrode active material. It is preferable to contain 0.1-1 weight% preferably. If it is the said range, an addition effect can fully be acquired and there is no possibility that the energy density of a positive electrode may fall. In the non-aqueous electrolyte containing the silyl ester compound, such as the non-aqueous electrolyte of the present terminology, electrolytic solution decomposition gas may be generated when stored at a high temperature of 60 ° C. or higher, and there may be a problem such as an increase in the thickness of the battery. Therefore, the above three kinds of lithium salts are present in the electrode mixture of the positive electrode or on the surface, whereby the possibility of gas generation is further reduced, and a lithium ion secondary battery in which the increase in battery thickness is less likely to occur even after high temperature storage can be obtained.

Such an effect can be obtained when the silyl ester compound decomposes in the nonaqueous electrolyte of the present invention, and a component that promotes the generation of cracked gas is likely to be produced, but the above three types of lithium salts do not produce the cracked gas. It is considered to have a function of absorbing a component to be accelerated, whereby decomposition gas generation is suppressed.

As a method of containing lithium phosphate, lithium sulfate, and lithium fluoride in a positive electrode, i) lithium phosphate, sulfuric acid in a positive electrode mixture layer (a mixture of a positive electrode active material formed on a positive electrode current collector, a binder, and a conductive assistant such as carbon black) Method for containing one or two or more powders selected from lithium and lithium fluoride, pellets, whisker fibers and the like, ii) A solution in which one or two or more selected from lithium phosphate, lithium sulfate and lithium fluoride are dissolved or dispersed Is applied to the positive electrode and then the solvent is removed. Iii) A method of producing a positive electrode using a positive electrode active material and a conductive aid in which at least one selected from lithium phosphate, lithium sulfate and lithium fluoride is formed on the surface thereof. , iv) pouring into a lithium ion secondary battery in the state of dispersing one or two or more selected from lithium phosphate, lithium sulfate and lithium fluoride in the nonaqueous electrolyte of the present invention, And a method of depositing from the Zheng Keshuang of the electrolytic solution. Lithium phosphate, lithium sulfate, and lithium fluoride are hardly dissolved in the nonaqueous electrolyte, so the method of iii) is most preferred in order to increase the effective surface area. In addition, when performing the method of i), ii), and iv), it is preferable to use what has a large surface area and a fine particle diameter.

The separator electrically insulates the positive electrode and the negative electrode and may use a porous membrane, a polymer electrolyte, or the like as a membrane that transmits lithium ions. As the porous membrane, a microporous polymer film is preferable, and the material thereof is polyolefin, polyimide, polyvinylidene fluoride, polyester or the like. Porous polyolefin films are particularly preferred, and specific examples thereof include porous polyethylene films, porous polypropylene films, multilayer films of porous polyethylene films and polypropylene, and the like. Another resin having excellent thermal stability may be coated on the porous polyolefin film. Examples of the polymer electrolyte include polymers in which lithium salts such as lithium hexafluorophosphate are dissolved, and crosslinked polymers swelled in an electrolyte solution. You may use the nonaqueous electrolyte solution of this invention for the purpose of obtaining a polymer electrolyte.

The lithium ion secondary battery of this invention can be made into arbitrary shapes, for example, is formed in cylindrical shape, coin shape, square shape, film shape, etc. However, the basic structure of the battery is the same regardless of the shape, and it is possible to change the design according to the purpose.

For example, in the cylindrical lithium ion secondary battery of the present invention, the non-aqueous electrolyte solution of the present invention is impregnated by enclosing the sheet-shaped negative electrode and the sheet-shaped positive electrode through a separator, and the enclosing body is mounted so that the insulating plate is placed above and below. It is a structure housed in a battery can.

In addition, the coin-type lithium ion secondary battery of the present invention has a structure in which an electrolyte solution is impregnated into a laminate of a disc-shaped negative electrode, a separator, and a disc-shaped positive electrode, and a spacer plate is inserted, if necessary, in a coin-type battery can. have.

The lithium ion secondary battery of the present invention can be used for the same use as a conventional lithium ion secondary battery. For example, various kinds of consumer electronic devices, especially mobile phones, mobile phones, laptop personal computers, cameras, portable video recorders, portable CD players, portable MD players, hybrid electric vehicles, night power storage, etc. Can be used.

EXAMPLE

Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples.

(Examples 1-8 and Comparative Examples 1-5)

[Resolution of thickening caused by silyl ester compound by tetrafluoroborate]

Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio EC: EMC = 4: 6, and lithium hexafluorophosphate was dissolved (concentration 1 mol / liter in the nonaqueous electrolyte) to prepare an electrolyte solution. The non-aqueous electrolyte solution of this invention and the comparative example was prepared by adding the silyl ester and tetrafluoroborate to the said electrolyte solution so that it may become the ratio of Table 1 (weight%, abbreviated as "%" in Table 1 simply) with respect to gross weight.

The viscosity and conductivity of the obtained nonaqueous electrolyte were measured at 25 ° C using an E-type viscometer (manufactured by Tokiko Co., Ltd.) and a conductivity meter (trade name: CM-40S, manufactured by Toyo Denki Co., Ltd.). The results are written together in Table 1. Table 1 also shows the results when only tetrafluoroborate is added as a reference example.

In addition, the viscosity change of the non-aqueous electrolyte when the content of the lithium tetrafluoroborate salt is 0.2% by weight and the content of the tris (trimethylsilyl) phosphate is changed between 0.5 to 3% by weight, does not contain lithium tetrafluoroborate. 1 is shown with an example.

In Examples of the present invention in which both the silyl ester compound and the tetrafluoroborate are added from Table 1 and FIG. 1, the thickener and ion of the electrolyte solution were used in the comparative example in which only the silyl ester compound participated in the case where the thickening and the corresponding decrease in the ionic conductivity did not occur. It was found that a drop in conductivity was occurring.

Table 1

Silyl ester compound Tetrafluoroborate Viscosity mPacm Conductivity mS / cm Kinds % Kinds % Example One Tristrimethylsilyl Phosphate 0.5 Lithium tetrafluoroborate 0.2 3.4 8.9 2 Same as above 1.0 Same as above 0.2 3.4 8.9 3 Same as above 2.0 Same as above 0.2 3.4 8.9 4 Same as above 3.0 Same as above 0.2 3.4 8.8 5 Same as above 1.0 Same as above 0.5 3.5 8.8 6 Same as above 2.0 Sodium tetrafluoroborate 0.2 3.5 8.9 7 Same as above 2.0 Tetraethylammonium Tetrafluoride 0.2 3.5 8.9 8 Di (trimethylsilyl) sulfate 2.0 Lithium tetrafluoroborate 0.2 3.4 8.9 Comparative example One Tristrimethylsilyl Phosphate 0.5 - - 3.7 8.8 2 Same as above 1.0 - - 3.9 8.7 3 Same as above 2.0 - - 4.4 8.4 4 Same as above 3.0 - - 4.9 8.1 5 Di (trimethylsilyl) sulfate 2.0 - - 3.7 8.7 Reference Example - - Lithium tetrafluoroborate 0.2 3.4 8.9

(Examples 9-22 and Comparative Examples 6-8)

[Effect of Reducing Interfacial Resistance by Use of Silyl Ester Compound and Fluoride Borate]

Evaluation of interfacial resistance was performed by producing the coin type lithium ion secondary battery shown below.

1) Preparation of electrolyte

Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio EC: EMC = 4: 6, and lithium hexafluorophosphate was dissolved (concentration 1 mol / liter in the nonaqueous electrolyte) to prepare an electrolyte solution. To the electrolyte solution, a silylene ester compound and tetrafluoroborate are added so that vinylene carbonate is 1% by weight relative to the total weight, and then to the ratio shown in Table 2 (weight%, simply abbreviated as "%" in Table 2). The nonaqueous electrolyte solution of this invention and a comparative example was prepared.

TABLE 2

Silyl ester compound (A) Tetrafluoroborate (B) B / A Kinds % Kinds % Example 9 Tristrimethylsilyl Phosphate 1.0 Lithium tetrafluoroborate 0.1 0.1 10 Same as above 2.0 Same as above 0.5 0.25 11 Same as above 1.5 Same as above 0.5 0.33 12 Same as above 1.0 Same as above 0.5 0.5 13 Same as above 0.5 Same as above 0.3 0.6 14 Same as above 0.5 Same as above 0.5 One 15 Di (trimethylsilyl) sulfate 0.5 Same as above 0.5 One 16 Tristrimethylsilyl Phosphate 0.5 Tetraethylammonium Tetrafluoride 0.5 One 17 Same as above 0.35 Lithium tetrafluoroborate 0.5 1.4 18 Same as above 0.5 Same as above 1.0 2 19 Same as above 1.0 Same as above 2.0 2 20 Same as above 0.5 Same as above 2.0 4 21 Same as above 0.3 Same as above 2.0 6.7 22 Same as above 0.15 Same as above 2.0 13.3 Comparative example 6 - - - - - 7 - - Lithium tetrafluoroborate 1.0 - 8 Tristrimethylsilyl Phosphate 1.0 - - -

2) Production of negative electrode

74 parts by weight of mesocarbon microbead (brand name: MCMB 10-28, Osaka Gas), 20 parts by weight of natural graphite (trade name: LF18A, 中 越 黑 越) and polyvinylidene fluoride (PVDF) 6) parts by weight were mixed and dispersed in 100 parts by weight of N-methylpyrrolidinone to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to a strip-shaped copper foil negative electrode current collector having a thickness of 18 μm, and dried. This was compressed with a roll press to obtain a negative electrode. In the negative electrode, the packing density of the active material was 1.5 g / ml.

3) production of positive electrode

82 parts by weight of LiCoO ₂ (trade name: HLC-22, manufactured by FMC Energy Systems), 7 parts by weight of graphite (conductor) and 3 parts by weight of acetylene black (conductor) and polyvinylidene fluoride (PVDF, binder) (8) 8 parts by weight were mixed and dispersed in 80 parts by weight of N-methylpyrrolidone to prepare a LiCoO ₂ mixture slurry. This LiCoO ₂ mixture slurry was applied to an aluminum foil (positive electrode current collector) having a thickness of 20 μm and dried. This was compressed by a roll press to obtain a positive electrode.

4) Production of coin type battery

As the negative electrode, the negative electrode obtained in the above 2) was used as a circular hole having a diameter of 14 mm on the bone. This negative electrode had a thickness of the negative electrode mixture of 80 μm and a weight of 20 mg / 14 mmφ.

The positive electrode obtained in the above 3) was used as a positive electrode in a circular shape having a diameter of 13.5 mm on the bone. This LiCoO ₂ electrode had a thickness of 70 μm and a weight of 42 mg / 13.5 mm φ of the LiCoO ₂ mixture.

A negative electrode (diameter 14 mm) is disposed on the negative electrode can surface in a 2032 size stainless steel battery can so as to contact the negative electrode current collector, and then a separator (thickness 25 μm, diameter 16 mm) and a positive electrode (diameter 13.5 mm) made of a microporous polypropylene film. ) Were stacked one by one. Thereafter, 0.04 ml of the nonaqueous electrolyte obtained in 1) above was introduced into the separator, and an aluminum plate (thickness 1.2 mm, diameter 16 mm) and a spring were accommodated. Finally, the airtightness in a battery was maintained by attaching the battery can lid via the gasket made from polypropylene, and the coin type lithium ion secondary battery of this invention and a comparative example which is 20 mm in diameter and 3.2 mm in height was produced.

The coin-type lithium ion secondary battery obtained as described above was charged to 4.2V and discharged to 3.0V at a current of 0.5mA. The discharge capacity at this time was made into the initial discharge capacity. Subsequently, after charging to 4.1V, 7 days are preserve | saved at 45 degreeC, After that, the impedance is measured using the frequency response analyzer (brand name: 1255B, the product made by Solartron), and 250 Hz from the 1 Hz inductance real part as a magnitude of interface resistance. The value obtained by subtracting the impedance real part was obtained. The results are shown in Table 3.

TABLE 3

Surface ResistanceΩ Example 9 3.1 Example 10 3.1 Example 11 3.0 Example 12 3.0 Example 13 2.9 Example 14 2.7 Example 15 2.7 Example 16 2.7 Example 17 2.8 Example 18 2.5 Example 19 2.4 Example 20 2.6 Example 21 2.8 Example 22 2.9 Comparative Example 6 3.2 Comparative Example 7 3.8 Comparative Example 8 2.8

From Table 3, it was found that the nonaqueous electrolyte solution of the examples to which the silyl ester compound and the tetrafluoroborate were added had smaller interfacial resistance than the nonaqueous electrolyte solutions of Comparative Examples 6 and 7.

(Examples 23 to 36 and Comparative Examples 9 to 11)

[Improvement of Load Characteristics by Use of Silyl Ester Compound and Fluoride Borate]

The same coin-type batteries as used in Examples 9 to 22 and Comparative Examples 6 to 8 were charged to 4.2 V, discharged to 3.0 V at a current of 10 mA, and the discharge capacity at 10 mA was obtained. The percentage of the discharge capacity at 10 mA to the initial discharge capacity was used as a load characteristic index and evaluated. The initial discharge capacity was about 4.5 mAh. The results are shown in Table 4.

From Table 4, it was found that the nonaqueous electrolyte of the example containing the silyl ester compound and the tetrafluoroborate had better load characteristics than the nonaqueous electrolyte of the comparative example. That is, in Examples 23-36, since the viscosity increase of the electrolyte solution by a silyl ester compound is eliminated, the effect | action of reducing the interface resistance of a silyl ester is fully exhibited, and the load characteristic is improved.

On the other hand, the non-aqueous electrolyte of Comparative Example 11 containing only tristrimethylsilyl phosphate has a poor load characteristic index, despite the excellent interfacial resistance as can be seen from the results of Comparative Example 8. As can be seen from the result of Comparative Example 2, this is considered to be caused by the increase in the viscosity of the nonaqueous electrolyte and the decrease in the ionic conductivity.

Table 4

Silyl ester compound Tetrafluoroborate Load characteristic index% Kinds % Kinds % Example 23 Tristrimethylsilyl Phosphate 1.0 Lithium tetrafluoroborate 0.1 72 24 Same as above 2.0 Same as above 0.5 74 25 Same as above 1.5 Same as above 0.5 74 26 Same as above 1.0 Same as above 0.5 74 27 Same as above 0.5 Same as above 0.3 74 28 Same as above 0.5 Same as above 0.5 75 29 Di (trimethylsilyl) sulfate 0.5 Same as above 0.5 74 30 Tristrimethylsilyl Phosphate 0.5 Tetraethylammonium Tetrafluoride 0.5 72 31 Same as above 0.35 Lithium tetrafluoroborate 0.5 73 32 Same as above 0.5 Same as above 1.0 77 33 Same as above 1.0 Same as above 2.0 78 34 Same as above 0.5 Same as above 2.0 75 35 Same as above 0.3 Same as above 2.0 74 36 Same as above 0.15 Same as above 2.0 74 Comparative example 9 - - - - 69 10 - 1.0 Lithium tetrafluoroborate - 70 11 Tristrimethylsilyl Phosphate - - 1.0 62

(Examples 37-46 and Comparative Examples 12-14)

[Improvement of High Temperature Storage Characteristics by Combined Use of Silyl Ester and Fluoride]

The same coin-type batteries as used in Examples 12, 14 to 22, and Comparative Examples 6 to 8 were charged to 4.2 V, discharged to 3.0 V at a current of 5 mA, and the discharge capacity at 5 mA before storage was obtained. After charging this battery to 4.2V, it stored at 85 degreeC for 3 days. After that, the battery was charged to 4.2V, discharged to 3.0V again at a current of 5mA, and the discharge capacity at 5mA before storage was obtained. The percentage of the discharge capacity at 5 mA after storage relative to the discharge capacity at 5 mA before storage was determined, and evaluated as a high temperature storage index. The results are shown in Table 5.

From Table 5, it was found that the nonaqueous electrolyte of the example containing the silyl ester compound and the tetrafluoroborate was superior in the high temperature storage characteristics compared to the nonaqueous electrolyte of the comparative example. That is, in Examples 37 to 46, the inclusion of the silyl ester compound eliminates the deterioration of the high temperature storage characteristics due to the fluoroborate, and rather improves the comparative example 12.

On the other hand, the nonaqueous electrolyte of Comparative Example 13 containing only lithium tetrafluoroborate has poor high temperature storage characteristics. This is considered to be because lithium tetrafluoroborate tends to react with the negative electrode and the irreversible self discharge during high temperature storage becomes large.

Table 5

Silyl ester compound Tetrafluoroborate High Temperature Storage Index% Kinds % Kinds % Example 37 Tristrimethylsilyl Phosphate 1.0 Lithium tetrafluoroborate 0.5 90.0 38 Same as above 0.5 Same as above 0.5 90.1 39 Di (trimethylsilyl) sulfate 0.5 Same as above 0.5 90.0 40 Tristrimethylsilyl Phosphate 0.5 Tetraethylammonium Tetrafluoride 0.5 90.0 41 Same as above 0.35 Lithium tetrafluoroborate 0.5 90.1 42 Same as above 0.5 Same as above 1.0 90.3 43 Same as above 1.0 Same as above 2.0 90.3 44 Same as above 0.5 Same as above 2.0 90.2 45 Same as above 0.3 Same as above 2.0 90.1 46 Same as above 0.15 Same as above 2.0 90.1 Comparative example 12 - - - - 89.9 13 - - Lithium tetrafluoroborate 1.0 87.2 14 Tristrimethylsilyl Phosphate 1.0 - - 88.8

(Examples 47 to 49 and Comparative Example 15)

[Inhibition Effect of Increase of Electrolyte Decomposition Gas at Preservation of High Temperature by Silyl Ester Compound]

Evaluation of the electrolyte decomposition gas at the time of high temperature storage produced the laminated type lithium ion secondary battery shown below, after charging at 4.2V, performing high temperature storage at 85 degreeC for 3 days, and evaluated from the volume change of the laminated battery before and after storage. .

1) Production of positive electrode

0.8 parts by weight of lithium fluoride finely ground in a mortar, 8 parts by weight of powdered LiCoO ₂ (trade name: HLC-22, manufactured by FMC Energy Systems), 7 parts by weight of graphite (conductive agent), acetylene black ( 3 parts by weight of the conductive agent) and 8 parts by weight of polyvinylidene fluoride (PVDF, binder) were mixed and dispersed in 80 parts by weight of N-methylpyrrolidone to prepare a LiCoO ₂ mixture slurry. This LiCoO ₂ mixture slurry was applied to an aluminum foil (positive electrode current collector) having a thickness of 20 μm and dried. This was compressed by a roll press to obtain a lithium fluoride-containing positive electrode of Example 47.

In the same manner, a lithium positive electrode was used in place of lithium fluoride or lithium lithium sulphate to prepare a lithium phosphate-containing positive electrode (Example 48) and a lithium lithium sulfate-containing positive electrode (Example 49). An additive-free positive electrode (Comparative Example 15) was produced in the same manner except that none of lithium fluoride, trilithium phosphate, and lithium lithium sulfate was included.

2) Fabrication of Laminate Battery

Using the same negative electrode produced in Example 9, the negative electrode having a size of 85 mm x 50 mm and the positive electrode having a size of 76 mm x 46 mm were cut out and faced with a separator made of a microporous polypropylene film having a width of 55 mm and a length of 110 mm. I made it to the military. This electrode group was accommodated in a cylindrical bag made of an aluminum laminate film so that both the lead wires of the positive electrode and the negative electrode could be drawn out from one of the opening portions, and first, the side from which the lead wire was drawn was closed by heat sealing. Subsequently, 1.0 ml of the nonaqueous electrolyte solution was injected into the electrode group and impregnated, and then the remaining open portion was heat-sealed to seal the electrode group in a bag to obtain a laminate battery.

The nonaqueous electrolyte was prepared by mixing EC and EMC in a weight ratio of EC: EMC = 4: 6, and dissolving lithium hexafluorophosphate therein (concentration 1 mol / liter in the nonaqueous electrolyte) to vinylene carbonate relative to the total weight. Is added to 1% by weight, and in Examples 47 to 49, 0.5% by weight of tristrimethylsilyl phosphate and 1% by weight of lithium tetrafluoroborate are added, and 0.5% by weight of only tristrimethylsilyl phosphate in Comparative Example 15 It was added so that it might prepare nonaqueous electrolyte solution, respectively.

3) Measurement of volume change of battery

After charging this laminated battery at 4.1V and storing (referring to aging) at 45 degreeC for 24 hours and 7 days, charge / discharge was performed from 4.2V to 3.0V, and the battery capacity was confirmed. At this time, the battery had a capacity of 150 mAh. Subsequently, this battery was charged to 4.2V and stored at 85 ° C for 3 days at high temperature. The battery volume after aging and the battery volume after high temperature storage were measured by the Archimedes method, and the battery expansion at the time of high temperature storage was calculated | required from the difference. The results are shown in Table 6.

Table 6

Positive additive Battery expansion (ml) Example 47 Lithium fluoride 0.75 Example 48 Trilithium Phosphate 0.65 Example 49 Dilithium sulfate 0.80 Comparative Example 15 - 1.0

Table 6 shows that the lithium ion secondary battery using the nonaqueous electrolyte solution of the present invention and containing a positive electrode to which lithium fluoride, triphosphate or lithium sulfate is added has been suppressed from expanding due to electrolytic decomposition gas at high temperature storage. okay.

(Examples 50-53 and Comparative Examples 16-18)

[Effect of Improvement of Cycle Characteristics by Combination of Silyl Ester Compound and Borofluoride]

Using the coin-type battery used in Example 9, charge and discharge cycle tests at 0 ° C and 25 ° C were performed. The charging condition is to charge 4.2V at a constant current of 3mA, and then maintain 4.2V until the current value becomes 0.3mA, and the discharge condition is to discharge from 5mA to 3.0V. The discharge capacity was measured.

The nonaqueous electrolyte filled in the coin-type battery was mixed with EC and EMC in a weight ratio EC: EMC = 4: 6, and dissolved in lithium hexafluorophosphate (1 mol / liter in non-aqueous electrolyte) to the electrolyte solution obtained. The vinylene carbonate was added in an amount of 1% by weight based on the total weight, and then tritrimethylsilyl phosphate and lithium tetrafluoroborate were added in the ratio shown in Table 7 (weight%, abbreviated simply as "%" in Table 7). Used one.

Figure 2 is a graph showing the charge and discharge cycle test results at 25 ℃. The capacity retention ratio of the vertical axis represents the ratio (%) of the discharge capacity in each cycle to the discharge capacity of the first cycle.

2 shows that the coin-type batteries of Examples 50 to 53 do not have a wide drop in capacity retention rate even after 400 cycles, and have excellent charge / discharge cycle characteristics. On the other hand, the coin-type batteries of Comparative Examples 16 to 18, in particular, the coin-type batteries of Comparative Examples 16 and 18, sharply decreased in capacity retention after 300 cycles. It was found that the retention rate was insufficient.

TABLE 7

Silyl ester compound Tetrafluoroborate Kinds % Kinds % Example 50 Tristrimethylsilyl Phosphate 0.5 Lithium tetrafluoroborate 0.5 51 Same as above 0.35 Same as above 0.5 52 Same as above 0.5 Same as above 1.0 53 Same as above 1.0 Same as above 2.0 Comparative example 16 - - - - 17 - - Lithium tetrafluoroborate 1.0 18 Tristrimethylsilyl Phosphate 1.0 - -

(Examples 54 to 55 and Comparative Examples 19 to 21)

[Effect of Improving Cycle Characteristics at Negative Electrode with Higher Charge Density]

1) Fabrication of negative electrode with high filling density

Mesocarbon microbead (brand name: MCMB 10-28, Osaka Gas) 54 parts by weight, natural graphite (brand name: LF18A, 中 越 黑 鉛) 40 parts by weight and polyvinylidene fluoride (PVDF, binding 6) parts by weight were mixed and dispersed in 100 parts by weight of N-methylpyrrolidinone to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to a negative electrode current collector made of a band-like copper foil having a thickness of 18 μm, and dried. This was press-molded by a roll press once and the negative electrode which made the packing density of an active material 1.75 g / ml was obtained. Subsequently, this negative electrode was compression molded by two roll presses to obtain a negative electrode having a packing density of 1.65 g / ml of the active material. Table 8 shows the packing density of the negative electrode active material used in each of Examples and Comparative Examples.

2) Production of coin type battery

Using the said negative electrode, the coin type battery was produced by the method similar to Example 9.

The nonaqueous electrolyte filled in the coin-type battery was mixed with EC and EMC in a weight ratio EC: EMC = 4: 6, and dissolved in lithium hexafluorophosphate (1 mol / liter in non-aqueous electrolyte) to the electrolyte solution obtained. To the total weight of the stock solution, vinylene carbonate is added to 1% by weight, and then tristrimethylsilyl phosphate and lithium tetrafluorophosphate are added so as to have a ratio shown in Table 8 (weight%, abbreviated simply as "%" in Table 8). The added thing was used.

3) Charge / discharge cycle test

The charge / discharge cycle test was done at 25 degreeC using the coin type battery produced above. The charging condition is to charge 4.2V at a constant current of 3mA, and then maintain 4.2V until the current value becomes 0.3mA, and the discharge condition is to discharge from 5mA to 3.0V. The discharge capacity was measured. The results are shown in FIG. The capacity retention ratio of the vertical axis represents the ratio (%) of the discharge capacity in each cycle to the discharge capacity of the first cycle.

3, the coin-type batteries of Examples 54 and 55 filled with the nonaqueous electrolyte solution of the present invention containing both tritrimethylsilyl phosphate and lithium tetrafluoroborate exhibited a high capacity retention of about 80% even after 500 cycles. It was found that the degradation was small.

On the other hand, in Comparative Examples 19 to 21 filled with the non-aqueous electrolyte containing only tristrimethylsilyl phosphate or not containing both, it was found that after 300 cycles, the capacity retention ratio became remarkable and the cycle degradation was large. Further, from Comparative Example 20 and Comparative Example 21, it was found that increasing the packing density of the negative electrode further increased the cycle deterioration.

Table 8

Tristrimethylsilyl Phosphate Lithium tetraborate Negative Charge Density g / ml Example 54 0.5 1.0 1.55 Example 55 0.5 1.0 1.65 Comparative Example 19 - - 1.55 Comparative Example 20 0.5 - 1.55 Comparative Example 21 0.5 - 1.65

In the nonaqueous electrolyte solution for lithium batteries of the present invention, even in a lithium ion secondary battery having a high packing density of a negative electrode active material, and also a high energy density, reduction and decomposition of the electrolyte solvent in the negative electrode are prevented, and the interface resistance between the negative electrode or the positive electrode and the electrolyte solution is increased. In addition, there is little deterioration in the lithium ion conductivity due to thickening, and furthermore, since such desirable characteristics are maintained for a long time, it is possible to further improve the load characteristics, low temperature characteristics, and high temperature storage characteristics of the batteries while maintaining the charge / discharge cycle characteristics of the batteries at a high level. Can be.

The lithium ion secondary battery containing the nonaqueous electrolyte solution for lithium batteries of the present invention has a high energy density, exhibits a high level of charge / discharge cycle characteristics over a wide temperature range from low temperature to high temperature, and has excellent load characteristics and low temperature characteristics.

In addition, the lithium ion secondary battery of the present invention hardly causes deterioration of the characteristics of the non-aqueous electrolyte even when stored for a long time or stored at a high temperature of 60 ° C. or higher, so that the charge and discharge cycle characteristics, the load characteristics, and the low temperature characteristics do not deteriorate to the point of practical use. Excellent preservation safety.

Claims (5)

A non-aqueous electrolyte containing a lithium salt and a non-aqueous solvent, further comprising a silyl ester compound so as to contain 0.1 to 10% by weight of the total amount of the nonaqueous electrolyte and tetrafluoroborate to 0.01% by weight or more of the total amount of the nonaqueous electrolyte. Non-aqueous electrolyte for lithium batteries. The nonaqueous electrolyte solution for lithium batteries according to claim 1, wherein the weight ratio (content of tetrafluoroborate / content of silyl ester compound) of the silyl ester compound and tetrafluoroborate is 0.05 or more. The non-aqueous electrolyte for lithium batteries according to claim 1, wherein the silyl ester compound is trimethylsilyl phosphate. The nonaqueous electrolyte solution for lithium batteries according to claim 1, wherein the lithium tetrafluoroborate is lithium tetrafluoroborate. A lithium ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte solution capable of occluding and releasing lithium, wherein the electrolyte solution is the nonaqueous electrolyte solution according to any one of claims 1 to 4.