JP2003242991A - Stabilized lithium electrochemical cell containing alkoxysilane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize the primary and secondary lithium batteries with respect to the performance and decomposition of the electrolyte used for these. <P>SOLUTION: An alkoxysilane compound as expressed by the general formula (R<SB>1</SB>O)<SB>n</SB>SiR<SB>4-n</SB>(wherein, R<SB>1</SB>expresses an alkyl group, R is selected from hydrogen, an alkyl group, alicyclic group, and aryl group, and each R<SB>1</SB>and R is mutually the same or different, and the alkyl group is straight chain or branch chain having carbon atom of less than 10, and n is an integer 1-3) is added approximately 0.01-5 wt.% to the non-aqueous electrolyte containing lithium salt dissolved in an organic solution and used in a primary and secondary lithium batteries. As a solvent, organic carbonate is desirable. As a lithium salt, LiPF<SB>6</SB>or the like which are in public domain can be used. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to improving the performance of lithium electrochemical cells and batteries. More particularly, the present invention relates to improved electrolytes containing siloxane additives for lithium-containing electrochemical cells and batteries.

[0002]

BACKGROUND OF THE INVENTION Lithium battery structures are manufactured from one or more lithium electrochemical cells. The cell is an anode,
It includes a cathode and an electrolyte that is separated from the positive electrode and the negative electrode and is electrically insulated and interposed. The electrolyte typically comprises a lithium salt dissolved in one or more solvents such as non-aqueous aprotic organic solvents. Lithium metal is a preferred anode material for batteries with excellent thermodynamic and kinetic properties. Lithium is a good conductor of electricity and heat. Lithium metal is flexible and malleable and can be easily extruded into sheets. However, it is well known that lithium is reactive with water and other reagents.

The anode typically comprises a nickel, iron, steel and / or copper foil current collector. The cathode also typically comprises an aluminum, nickel, iron, steel and / or copper foil current collector. Commercially available lithium cells with pure lithium anodes are currently used only as primary cells because of their activity.

As a result of this reactivity, several drawbacks can occur, such as an exothermic reaction and the formation of a passive film on the anode surface. In some environments, the generation of heat by exothermic reactions may result in an explosive release of energy. Exposing primary lithium batteries to temperatures above the recommended levels results in an exothermic reaction with intense heat generation.

Rechargeable, or secondary, cells are preferred over primary (non-rechargeable) cells because of the reversible chemistry involved in the positive and negative electrodes of the battery. The electrodes of the secondary cell can be recharged by applying a charge to it. Many advanced electrode systems have been developed to store charge. At the same time, much effort has been devoted to the development of electrolytes that can enhance the capacity and performance of electrochemical cells. The most practical electrolytes in terms of functionality, reasonable price, and ease of handling are organic solutions containing lithium ion salts. Although liquid electrolytes exhibit acceptable ionic conductivity, they must remove as many impurities as possible that would affect cell performance. Li-ion battery is LiAsF ₆ , LiCl
O ₄ , lithium triflate, LiBF ₄ or LiPF
Non-aqueous electrolytes containing salts such as ₆ and solvent mixtures such as ethylene carbonate, propylene carbonate, diethyl carbonate, etc. should be used to remove as completely as possible the impurities that may affect the performance of the cell. US Pat. No. 5,8, granted to Norang et al.
More specifically, as disclosed in US Pat. No. 30,600 (incorporated herein by reference), the rechargeability of a lithium ion electrode depends on side reactions of lithium ions and autocatalytic reactions within the cell. Limited. Lithium ion electrodes are constrained by side reactions between metallic lithium and impurities such as acids and moisture. Moisture may be preexisting or generated on its wall. The acid can also be generated on its wall by the reaction of water with the electrolyte component. When such impurities react with the lithium ion anode, a solid surface layer is formed on the lithium ion anode, increasing the impedance of the anode.

Due to the reactivity at the anode, an undesired reaction of impurities in the electrochemical cell with the cell constituents is substantially generated. US Pat. No. 5,419,985 to Kokshang describes the effect of impurities on lithium ion and lithium metal anodes when water reacts with lithium to form a high impedance solid surface layer. . Lithium-ion batteries that use graphite or carbon as the negative electrode are also susceptible to passivation at the carbon electrode by undesired reactions caused by the presence of impurities, especially moisture and acids.

Although liquid electrolytes exhibit acceptable ionic conductivity, they must be stripped of as many impurities as are likely to affect cell performance. The problem of lithium reactivity with electrolytes has been addressed in various ways. One method for reducing flammability and increasing stability, as disclosed in US Pat. No. 5,830,600, is to use phosphates, phosphorenes, cyclophospharenes, halogenated carbonates, fluorinated polysulfates. Dissolving the lithium salt in a solvent such as ethers or complex silanes and mixtures thereof. These solvents provide non-flammable, self-extinguishing electrolytes for lithium batteries. A large amount of silane and siloxane composite compound is required as a solvent. However, as described in the patent, such high amounts result in poor electrical performance, especially when used with carbon dioxide producing compounds.

Therefore, there is a need to provide more stable primary and secondary lithium batteries with respect to electrolyte performance and decomposition. The cell requires an electrolyte that is chemically stable with respect to the lithium component and that stops the corrosion of metal current collectors such as aluminum and steel that occurs with some lithium electrolytes.

[0009]

The present invention is directed to the selection of alkoxysilane compounds and the decomposition of one or more lithium electrochemical cell components including a lithium ion or lithium metal anode, a lithium intercalation compound cathode and a non-aqueous electrolyte. Provide a way to prevent. The addition of small amounts of low molecular weight alkoxy and / or dialkoxysilanes to the non-aqueous electrolyte effectively solves the problems that occur during the interaction of the cell components with the water present. The water reacts with the electrolyte containing the lithium salt in the organic solvent. This interaction of salt and water results in acid formation. The method of adding the selected alkoxysilane compound of the present invention to the electrolyte effectively inhibits the corrosive effect of the acid by producing a reaction product that is relatively inert to the action of the active material in the cell. . Preferably, the compound is dissolved in the electrolyte. As a result of this corrosion-reducing effect, the stability of the primary battery is increased and the capacity delivered from the secondary battery is improved even after long-term recycling. Rechargeable or secondary batteries usually exhibit a loss of capacity delivered as a function of charge / discharge cycles. Another unexpected advantage conferred by the alkoxysilane composition of the present invention on lithium electrochemical cells is that it normally appears to corrode aluminum current collectors, especially lithium hexaphosphate,
To enable the use of aluminum current collectors with certain lithium electrolyte additives such as lithium tetrafluoroborate, lithium trifluorate.

Alkoxysilane compounds useful as additives according to the present invention have the general formula: (R ¹ O) _n SiR _4-n (wherein R ¹ is an alkyl group, R is hydrogen, an alkyl group, an alicyclic group). Radicals and aryl groups, where each R ¹
And R components can be the same or different from each other, and the alkyl groups are straight or branched with less than 10 carbon atoms and n is an integer from 1 to 3). Particularly preferred compounds are trimethylethoxysilane, dimethyldiethoxysilane, methyltrimethoxysilane and dimethyldimethoxysilane. Preferably, the amount of alkoxysilane compound used in the electrolyte is about 0.25 wt% based on the total weight of the electrolyte. These small amounts are necessary to selectively react with the small amount of acid produced by the trace impurities present on the anode, cathode or in the electrolyte. Also, this small amount does not impair the electrical properties of the electrolyte.

The mono- and dialkoxysilane electrolyte additives of the present invention are chemically compatible with battery components. In other words, the alkoxysilanes are relatively inert with respect to components such as anodes, cathodes and current collectors. It has been found that an alkoxysilane additive compound can be used with lithium salt-based electrolytes that typically corrode aluminum current collectors, especially those containing lithium hexafluorophosphate and lithium tetrafluoroborate. Alkoxysilanes also prevent coloration of the electrolyte solution. The coloration indicates the presence of acid due to the interaction between the organic solvent and the acid.

The small amount of alkoxysilane additive compound incorporated into the electrolyte does not affect the bulk properties of the cell or battery. Dissolving the additive in the electrolyte makes the electrolyte easier to process during manufacture and is better dispersed throughout the cell. It should be noted that the use of too much additive compound may impair the thermal stability of the battery and cell. It may be expected that the excess amount of dissolved additive compound will adversely affect the conductivity and cycleability of the electrolyte.

The main object of the present invention is to provide an excellent lithium electrochemical cell or battery having excellent cycle characteristics and maintaining its integrity over a long life cycle.

Another object of the present invention is to provide a stabilized electrochemical cell which stabilizes against decomposition of cell components including electrodes and electrolyte components. A related specific object of the invention is primarily to prevent coloration of side reactions due to the reaction of acids with organic solvents of the electrolyte.

Another object of the present invention is to include an additive for use with an organic electrolyte which is neutralized with acid and / or moisture to produce a product which is relatively inert with respect to the active material of a lithium electrochemical cell. To provide.

A more particular object of the present invention is a critical amount of selected alkoxy and dialkoxy silanes which substantially reduce or completely eliminate the corrosive interactions of the various components of the lithium electrochemical cell. It is to provide a method for dissolving a compound.

These and other objects, features and advantages will be apparent from the following description of the preferred embodiments and claims.

[0018]

SUMMARY OF THE INVENTION According to the present invention, stabilized lithium electrochemical cells and cells are protected by inhibiting the decomposition of one or more electrochemical cell components. The method of the present invention effectively inhibits the decomposition of typical lithium electrochemical electrolytes containing lithium salts in non-aqueous aprotic organic solvents. Decomposition occurs due to the interaction of the lithium salt with the water that initially forms the acid that is present in or formed in these electrolyte-filled cells. Traces of acid generated by these interactions are treated by adding small amounts of low molecular weight alkoxysilanes. Although the exact chemical mechanism involved is not known, it is theorized that the neutralization reaction occurs to yield an inert hydrolysis product that can be enhanced without compromising the performance of the electrochemical cell. For example, some of the advantages of incorporating the additives of the present invention may increase the storage and service life of batteries and cells.
The service life is increased because the cycle characteristics are extended without reducing the cell capacity. Also, the effective temperature range is widened.

Low molecular weight alkoxysilane additives which are relatively inexpensive and which can be easily incorporated into the lithium salt-based electrolyte composition of an electrochemical cell have the general formula: (R ¹ O) _n SiR _4-n (In the formula, R ¹ is an alkyl group, and R is selected from hydrogen, an alkyl group, an alicyclic group and an aryl group, wherein each R is
¹ and R components can be the same or different from each other, and the alkyl group can be straight or branched with less than 10 carbon atoms and n can be an integer from 1 to 3). Particularly preferred compounds are trimethylethoxysilane and dimethyldiethoxysilane.

Small amounts of low molecular weight alkoxysilanes ranging from 0.01 to 3% by weight, based on the total weight of the electrolyte, are added. These small amounts are necessary to selectively react with the small amount of acid produced by trace impurities present on the anode, cathode or in the electrolyte. Also, the small amount does not affect the viscosity and, as mentioned above, does not impair the electrical properties of the electrolyte.

The mono- and dialkoxysilane electrolyte additives of the present invention are chemically compatible with battery components. In other words, the alkoxysilanes are relatively inert to components such as anodes, cathodes and current collectors.
It has been found that the alkoxysilane additive compounds can be used with lithium salt-based electrolytes that typically corrode aluminum current collectors, especially those containing lithium hexafluorophosphate and lithium tetrafluoroborate. Alkoxysilanes also prevent coloring of the electrolyte solution. The coloration indicates the presence of acid due to the interaction between the organic solvent and the acid.

The small amount of alkoxysilane additive compound mixed in the electrolyte does not affect the bulk properties of the cell or battery. When the additive is dissolved in the electrolyte, the electrolyte becomes
It is easy to process during manufacturing and is well dispersed throughout the cell. It should be noted that the use of excessive amounts of additive compounds can adversely affect the thermal stability of the batteries and cells. Excessive dissolved additive compound may be expected to adversely affect the conductivity and cycleability of the electrolyte.

As disclosed in US Pat. No. 5,830,600, significant amounts of silane and siloxane composite compositions are required when using these compositions as electrolyte solvents.

The term "lithium salt" according to the present invention is
formula: LiAN (In the formula, AN is Cl^-, CF₃, SO₃ ^-, ClO_Four ^-, BF
_Four ^-, Br^-, I^-, SCN ^-, AsF₆ ^-, N (CF₃S
O₂)₂ ^-, PF₆ ^-, SbF₆ ^-, O^-(CO) R_FourChoose from
Is an anion, and R ″ is especially alkyl,
Selected from reel, alkanyl, haloalkyl)
Including compound. The preferred lithium compound is LiPF₆,
LiAsF₆, LiBF₆Or LiCLO_FourThere is.

Organic solvents for use in lithium ion batteries include ethylene carbonate, propylene carbonate, dimethyl carbonate, dipropyl carbonate,
Dimethylsulfoxide, dimethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone and mixtures thereof include, but are not limited to. In a preferred embodiment, the dimethyl carbonate contains at least about 50% solvent. One preferred two component solvent system comprises ethylene carbonate / dimethyl carbonate.

Organic or inorganic thickeners such as synthetic organic polymers, silicone dioxides and aluminum oxides can be used to modify the liquid electrolyte solution to a gel or solid state. The electrolyte system can be dispersed in the polymer gel structure. The polymer gel structure can consist of one or more different polymers.

Suitable polymers include polyethylene, polypropylene, polytetrafluoroethylene, polyethylene-terephthalate, polystyrene, nylon, polyvinylidene fluoride, polyurethane, polyethylene oxide, polyacrylonitrile, polymethyl acrylate,
It can be selected from polyacrylamide, polyvinyl acetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate, any of the above copolymers, and mixtures thereof.

When using the electrolyte of the present invention, a "pacificatio" between the anode and the electrolyte is obtained.
n) ”films, ie films that exist as contact surfaces for the electrolyte, enhance the stability of the primary battery and extend the cycle life of the battery, can be initially formed.

It will be appreciated that the term "lithium ion battery" includes batteries containing a lithium / carbon anode and cathode.

[0030]

EXAMPLES The following examples are provided as an illustration of the advantages of the method of the present invention and illustrate how an electrolyte for a particular lithium battery and cell can be effectively prepared. Example 1 A 1 molar solution (14%) was made using 1: 1 ethylene carbonate / dimethyl carbonate and analyzed for moisture using Karl Fischer titration. (All material treatments were done in an inert atmosphere.) To each of eight identical Teflon® (FEP) bottles was added 0.25 wt% of each additive to be evaluated. Shake the solution, 2
It was analyzed after 4 hours. The solutions were analyzed after 28 and 60 days to see the effect of the additives in maintaining low acid and low color levels in the electrolyte. The test results are shown in the attached table. (The alkyl siloxane reacts with bases above pH 3, so the indicator bromophenol blue was used to analyze residual acid.) Comparative Example 2 LiClO ₄ and 1: 1 ethylene carbonate / dimethyl according to the method of Example 1. A 1.0 molar solution of the mixture with carbonate was prepared. The solution was analyzed for acid by base titration with bromophenol blue indicator. (Bromothymol blue indicator cannot be used because it will not change color until the silane additive is hydrolyzed during aqueous filtration.) Acid is calculated as HF ppm. The water content was measured by the Karl Fischer titration method.

100 ml aliquots of this solution were each placed in 250 ml FEP bottles under an argon atmosphere. Tetramethoxysilane was added to each bottle. No additive was added to one bottle, which was considered the normal level or standard without the additive of the present invention.

After 24 hours, HF was calculated for all solutions.
ppm and water ppm were analyzed. The solution was analyzed again after 30 days. Attention was also paid to color, as it shows harmful side reactions due to acid catalysis occurring in the solvent. Note that at least two aryl or alkyl groups must be on the silicon atom. In other words, tetramethoxysilane did not appear to work. R _4-n S
Only i (OR ') _n silane compounds (where n = 1, 2, or 3) act. Cyclic silanes having at least one carbon bonded to each silicon and at least one oxygen bonded to each silicon also work. These results show that without the silane additive of the present invention, coloration increases with the acid component of the electrolyte solution. The preferred silane additive first reduces the acid content and keeps it constant.

[0033]

[Table 1]

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Joel McCloskey             Fira, Pennsylvania, United States             Pemberton Street, Delphi             1930 F-term (reference) 5H024 FF14 FF15 FF18 FF20 FF38                       FF40 HH02                 5H029 AJ04 AJ05 AJ07 AJ13 AM02                       AM03 AM05 AM07 HJ02

Claims (11)

[Claims] 1. A non-aqueous electrolyte containing a lithium salt dissolved in an organic solvent, which is represented by the general formula: (R ¹ O) _n SiR _4-n (wherein R ¹ is an alkyl group and R is hydrogen). , An alkyl group, an alicyclic group and an aryl group, each R ¹ and R may be the same or different from each other, and the alkyl group is a straight chain or branched chain having less than 10 carbon atoms, and n can be an integer from 1 to 3),
An electrolyte comprising about 0.01 to 5 weight percent alkoxysilane. 2. The electrolyte according to claim 1, wherein the silane is selected from the group consisting of trimethylethoxysilane, dimethyldiethoxysilane, methyltrimethoxysilane, and dimethyldimethoxysilane. 3. The electrolyte according to claim 1, wherein the organic solvent contains at least one organic carbonate. 4. The electrolyte according to claim 3, wherein the organic carbonate is selected from the group consisting of ethylene carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate, ethylmethyl carbonate, and a mixture thereof. 5. The electrolyte according to claim 4, wherein the organic carbonate is an equal volume mixture of ethylene carbonate and dimethyl carbonate. 6. The electrolyte solution is LiPF ₆ , LiSC
N, LiASF ₆ , LiClO ₄ , LiBF ₄ , LiN
The electrolyte according to claim 1, comprising a lithium salt selected from the group consisting of (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiSbF ₆ and mixtures thereof. 7. The electrolyte according to claim ₆ , wherein the lithium salt is LiPF ₆ . 8. An electrochemical cell containing the electrolyte of claim 1. 9. A lithium primary battery containing the electrolyte according to claim 1. 10. A non-aqueous rechargeable lithium-ion battery containing the electrolyte according to claim 1. 11. An electrochemical cell containing the electrolyte of claim 1, wherein the alkoxysilane dissolved in the electrolyte is about 0.25 weight percent based on the total weight of the electrolyte. Target cell.