WO2015069854A1

WO2015069854A1 - Electrolyte solution for capacitors and batteries

Info

Publication number: WO2015069854A1
Application number: PCT/US2014/064276
Authority: WO
Inventors: W. Novis Smith; Joel R. Mccloskey; Jonathan Maeyer
Original assignee: Retriev Technologies Inc
Current assignee: Retriev Technologies Inc
Priority date: 2013-11-06
Filing date: 2014-11-06
Publication date: 2015-05-14
Anticipated expiration: 2016-05-06
Also published as: WO2015069853A1

Abstract

The present invention provide novel compounds and electrolyte solutions which can be used in capacitors and lithium batteries and which have a liquidus range of from about -65 to about 171 degrees C.

Description

ELECTROLYTE SOLUTION FOR CAPACITORS AND BATTERIES

Cross-Reference to Related Applications

This application claims priority to United States Application No. 14/073,401 , filed November 6, 2013, and issued on July 22, 2014, as U.S. Pat. No. 8,785,057, and to United States Application No. 14/073,302, filed November 6, 2013, the disclosure of each of which is incorporated herein by reference in its entirety for all purposes.

Field of the Invention

The present invention relates to novel electrolyte solutions for ultra capacitors and lithium batteries having liquidus ranges of from -65°C to 171°C which contain new

symmetrical and unsymmetrical organic carbonates.

Background of the Invention

The performance of ultra capacitor and lithium battery electrolytes at low temperature is a continuing problem since the conductivity of the electrolyte will go to zero if it freezes before a desired low temperature performance is achieved. Various blends of organic carbonates have been used along with the addition of ethers and low molecular weight esters to achieve low temperature (-60°C) freezing points of the mixed solvents containing lithium hexafluorophosphate for low temperature lithium-ion battery performance. The use of mixtures of ethylmethyl carbonate with dimethyl carbonate and small quantities of ethylene carbonate have allowed performance down to -20°C and even -30°C in some cases for lithium battery electrolytes. The use of tetrahydrofuran (THF) and methyl formate and methyl acetate and dimethyl ethylene glycol (glyme) or dimethoxy ethane (DME) has allowed some battery electrolytes to achieve -40°C or even -50°C performance. The problem is that the performance of these electrolytes at high temperatures such as >70°C causes high vapor pressures in the batteries with these volatile low boiling solvents. In the case of ultra capacitors based on organic electrolytes the situation is similar except most current ultra capacitor electrolytes are based on the use of acetonitrile (low boiling, with bp 82°C) containing tetraethylammonium tetrafiuoroborate.

These ultra capacitor electrolytes have an upper operating voltage limitation of 2.7 V. The use of capacitor electrolyte solvents based on the organic carbonates and containing tetralkylammonium tetrafluoroborates has also been limited because of solubility limitations when using propylene carbonate, or by low temperature performance when using mixtures containing ethylene carbonate. Propylene carbonate has been used in mixtures with other organic carbonates for capacitor electrolytes but this solvent also limits cell voltage to about 3 V and the solubility of the tetrafiuoroborate salt decreases rapidly on cooling and also results in low conductivity of the electrolyte at temperatures below -20°C. The use of ethylene carbonate (mp 35°C) with cyclic organic carbonate mixtures containing organic quaternary tetrafiuoroborate salts for capacitor use gives higher operating cell voltage in ultra capacitors, but these electrolytes freeze before -20°C is reached. Low temperature cycling performance (non-freezing) is desired (required for the use of ultra capacitor performance in vehicle performance down to - 30°C). In aircraft, the temperature desired is down to -40°C. At the same time these applications desire high temperature performance >70°C with low vapor pressure. This means that volatile solvents which are used for low temperature performance cause problems at the high end of the desired performance range.

U.S. Patent No. 6,535,373 to Smith, et al, which is herein incorporated by reference in its entirety for all purposes, relates to non-aqueous electrolyte solutions containing quaternary ammonium tetrafiuoroborate salts that can be used in the present invention. The solvent used in the patent is a nitrile solvent.

U.S. Patent No. 7,924,549 to Smith, et al., which is herein incorporated by reference in its entirety for all purposes, relates to carbon electrodes for capacitors with a high

concentration of tetrafiuoroborate salts in a non-aqueous aprotic solvent.

U.S. Patent No. 6,980,415 to Higono, et al, which is herein incorporated by reference in its entirety for all purposes, discloses an electrolyte for capacitors comprising dimethyl carbonate and a spiro tetrafiuoroborate salt. The solvent and the tetrafiuoroborate salt can be used in the present invention.

U.S. Patent Publication No. 20070194266 to Chiba, et al. which is herein incorporated by reference in its entirety for all purposes, discloses an electrolyte solution for electric double layer capacitors comprising quaternary ammonium salts and ethylene and propylene carbonate as an electrolyte.

An electrolyte solution which contained an unsaturated carbonate solvent of less than 8% by weight and a saturated solvent of less than 5% by weight has been reported (U.S. Patent Publication No. 2007/0224514 to Kotato et al.). The reason for the low amounts of these two solvent types is that "if the volume exceeds the upper limit an excessive amount of negative electrode coating may be formed and prevent the migration of lithium ions." Prior art electrolytes that have high ionic conductivity (resulting in high rate capability) are very flammable due to the inclusion of low boiling point flammable solvents such as dimethyl carbonate (DMC), diethyl carbonate, ethylmethyl carbonate (EMC) and

dimethoxyethane (DME). If high boiling point non-flammable glycol ethers are included to improve fire resistance, the resulting electrolytes have low ionic conductivity. When a high percentage (40% or more) of propylene carbonate (PC) or gamma-butyrolactone (GBL) is present to lower the flammability of the solvent system, the lithium-ion batteries with graphitic anodes fabricated therefrom are non-competitive in performance, due to reaction of the PC or GBL with the anode. This reaction causes a very high irreversible capacity loss and a sharp decline in capacity during cycling. Thus, there still exists a need for electrolyte solutions that are both fire resistant and capable of providing high performance in batteries, even at lower temperatures.

Summary of the Invention

According to one embodiment of the invention, there are provided novel compounds which are useful as solvents for capacitors and lithium acid battery electrolyte solutions. The compounds consist of novel symmetrical and unsymmetrical (asymmetrical) carbonates of the formula:

RO(C=0)OR¹

wherein R is selected from the group consisting of methyl, ethyl, isopropyl, CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, R"OCH₂CH₂- and R"-0-

(CH₂)_n-, wherein n is 2, 3 or 4 and R¹ is selected from the group consisting of CH₃OCH₂CH₂-,

CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, CH₃CH₂OCH₂CH₂- and R"-

0-CH₂CH₂-, wherein R" is methyl, ethyl, propyl or isopropyl.

According to another embodiment of the invention, the novel carbonates of the invention can be mixed with each other or with other organic cyclic or linear carbonates, ethers and esters in amounts up to 80% by weight or up to 65% by weight of the mixed solvent systems.

One aspect of the invention provides novel organic carbonates.

According to another object of the invention there is provided an electrolyte solution for capacitors and lithium batteries comprising the novel organic carbonates alone or admixture with other suitable solvents for capacitors and lithium batteries. In another aspect of the invention, capacitors are provided with electrolyte solutions that contain soluble quaternary ammonium tetrafluoroborate salts and that have a wide liquidus range preferably from -70°C to >150°C.

In yet a still further embodiment of the invention, an ultra capacitor is provided having a stable operating cell voltage of about 4V when containing 2M of a quaternary ammonium tetrafluoroborate salt at room temperature.

One aspect of the invention provides an electrolyte solution for low temperature capacitors or lithium batteries comprising a solvent system which is comprised of, or which consists essentially of, or consists of 20-100% (alternatively, 35-100%) or 50-100%) by weight in total of at least one symmetrical or asymmetrical carbonate of the general formula (I):

RO(C=0)OR^l ( I )

wherein R is selected from the group consisting of methyl, ethyl, isopropyl, propyl,

CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-,

R"OCH₂CH₂- and R"-0-(CH₂)_n-, wherein n is 2, 3 or 4, R¹ is selected from the group consisting of CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, CH₃CH₂OCH₂CH₂- and R -0-CH₂CH₂-, and R" is methyl, ethyl, propyl or isopropyl and up to 80%) by weight, up to 65% by weight or up to 50% by weight in total of at least one solvent which is different from the symmetrical or asymmetrical carbonate of general formula (I), said electrolyte solution additionally comprising at least one conductive salt such as a lithium salt or a quaternary ammonium compound and having a liquidus range of about -65°C to about 171°C.

In one particularly desirable embodiment of the invention, the solvent system is comprised of 20 to 90 % by weight methyl-(2-methoxyethyl)-carbonate with the balance to 100%) being ethylene carbonate, subject to the proviso that optionally up to 5% in total of one or more carbonates other than methyl-(2-methoxyethyl)-carbonate and ethylene carbonate may be additionally present in the solvent system. Such a solvent system may be combined with one or more lithium salts, at a concentration of lithium salt of 0.5 to 2.5M, to provide an electrolyte solution.

Additionally provided by the present invention is a method of making methyl-(2- methoxyethyl)-carbonate, comprising the steps of:

a), reacting 2-methoxy ethanol and dimethyl carbonate in the presence of sodium methoxide to obtain an equilibration reaction mixture comprised of methanol, methyl- (2-methoxyethyl)-carbonate and bis-2-methoxyethyl carbonate; b) . removing methanol from the equilibration reaction mixture by distillation;

c) . neutralizing the equilibration reaction mixture from which methanol has been removed with an acid or acid salt to obtain a neutralized reaction mixture; and d) . removing methyl-(2-methoxyethyl)-carbonate from the neutralized reaction mixture by distillation.

Description of the Preferred Embodiments

According to the present invention, one embodiment relates to a novel solvent of symmetrical and unsymmetrical carbonates having the general formula

RO(C=0)OR¹

wherein R is selected from the group consisting of methyl, ethyl, isopropyl and propyl and taken from the group CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH2-, and

CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, and R"OCH₂CH₂- and R"-0-(CH₂)_n-, wherein n is 2, 3 or 4 and R¹ is selected from the group consisting of CH₃OCH₂CH₂-,

CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, CH₃CH₂OCH₂CH₂- and R -O- CH₂CH₂-, and wherein R" is methyl, ethyl, propyl or isopropyl.

According to another embodiment of the invention the carbonate solvents of the invention corresponding to the above-mentioned general formula may be a mixture of themselves in a total amount of 35 to 100% with up to 65% by weight (e.g., 0-65% by weight) of one or more other linear or cyclic carbonates, carboxylic esters and/or ethers. That is, the solvent can comprise 0-50%> by weight of cyclic carbonates, for example, 50% by weight EC and 50%) by weight of one or more carbonates of the invention, or, in another example, 25% by weight EC, 10% by weight PC and 65% by weight of one or more carbonates of the invention.

Cyclic carbonates include ethylene carbonate (hereafter designated as EC) and propylene carbonate (PC). Linear carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), etc.

The carboxylic ester optionally used herein preferably has three or more carbon atoms and one or more carboxylic ester bonds. The upper limit of the number of carbon atoms in the carboxylic ester is not particularly limited. In view of the compatibility with the chain carbonate and/or cyclic carbonate, the carboxylic ester preferably has ten or less carbon atoms and more preferably eight or less carbon atoms.

The number of carboxylic ester bonds in the carboxylic ester is one or more as described above. Since an increase in the number of carboxylic ester bonds generally leads to an increase in the viscosity of the carboxylic ester, the number of carboxylic ester bonds is preferably one or two.

Examples of suitable carboxylic esters include dimethyl succinate, ethyl methyl succinate, diethyl succinate, dimethyl 2-methylsuccinate, ethyl methyl 2-methylsuccinate, dimethyl glutarate, ethyl methyl glutarate, diethyl glutarate, dimethyl 2-methylglutarate, ethyl methyl 2-methylglutarate, diethyl 2-methylglutarate, dimethyl adipate, ethyl methyl adipate, diethyl adipate, 1 ,2-diacetoxyethane, 1 ,2-diacetoxypropane, 1 ,4-diacetoxybutane, glycerin triacetate, methyl 4-acetoxybutyrate, gamma butyrolactone, methyl 2-acetoxyisobutyrate, ethyl acetate and methyl acetate.

The electrolyte solutions of the present invention for capacitors and lithium batteries comprise the novel solvents of the invention and one or more conductive salts such as a lithium salt or a quaternary ammonium salt dissolved as an electrolyte.

The conductive salt may be any that is capable of being used in electrical storage devices, such as lithium secondary cells, lithium ion secondary cells and electrical double-layer capacitors. Conductive salts that may be used include alkali metal salts and quaternary ammonium salts. Combinations and mixtures of different conductive salts may be utilized.

Preferred conductive salts are lithium salts. Specific examples include lithium salts such as the lithium salt of bis(trifluoromethane sulfonyl) imide, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium trifluoromethanesulfonate (triflate), sulfonyl imide lithium salts, sulfonyl methide lithium salts, lithium acetate, lithium

trifluoroacetate, lithium benzoate, lithium p-toluenesulfonate, lithium nitrate, lithium bromide, lithium iodide and lithium tetraphenylborate.

Highly conductive quaternary ammonium and related imidazolium salts and triflates and mixtures thereof as tetrafluoroborates have synergistic effect on their solubilities and conductivities at low temperatures when dissolved in the solvents of the invention.

Specific examples of the quaternary ammonium salts suitable for use in the present invention include, but are not limited to, quaternary ammonium tetrafluoroborates and quaternary ammonium triflates such as triethylmethylammonium tetrafluoroborate, diethyldimethylammonium tetrafluoroborate, ethyltrimethylammonium tetrafluoroborate, dimethylpyrrolidinium tetrafluoroborate, diethylpyrrolidinium tetrafluoroborate,

ethylmethylpyrrolidinium tetrafluoroborate, spiro-(l,l ¹)-bipyrrolidinium tetrafluoroborate, dimethylpiperidinium tetrafluoroborate, diethyl piperidinium tetrafluoroborate, spiro-(l,l ¹)- bipiperidinium tetrafluoroborate, piperidine-l-spiro-1 ^pyrrolidinium tetrafluoroborate, or the tri Hates thereof.

Of these, triethylmethylammonium tetrafluoroborate, spiro-(l ,l ¹)-bipyrrolidinium tetrafluoroborate, diethyl pyrrolidinium tetrafluoroborate, dimethyl pyrrolidinium

tetrafluoroborate, and the like are particularly preferable.

The concentration of the conductive salt in the electrolyte solution of the present invention is preferably from 1 to 3 mol/1, particularly preferably from 1 M to 2 M.

I the concentration of the conductive salt is less than 0.5 mol/1, the conductivity may be insufficient; if more than 3 mol/1, the low temperature performance and economical efficiency may be impaired.

What is needed are solvents with high operating voltages greater than 3 V for the ultra capacitor in which conductive salts such as quaternary tetrafluoroborate salts are very soluble and which have a wide liquidus range preferably from -70°C to >150°C. In addition, the solvent should remain liquid with high salt concentrations of salt from >1 M up to 2 M. The same is also true for lithium ion battery electrolytes. The organic carbonates of the invention meet all of these requirements. The preferred solvent is methyl methoxyethyl carbonate (CH30(C=0)OCH₂CH₂OCH3), (MMC). The liquidus range of this solvent is about -65°C to about 171°C providing outstanding low temperature and high temperature performance while dissolving a high level of conductive salt. This solvent has a stable operating cell voltage of about 4 V in ultra capacitor performance when containing 2 M diethyldimethyl ammonium tetrafluoroborate at room temperature. Other quaternary alkylammonium tetrafluoroborates can also be used for this same stable high operating voltage in this solvent and related solvents. This same stable solvent with 2 M quaternary ammonium tetrafluoroborate salt will reversibly charge/discharge cycle at -45°C in an ultra capacitor.

The electrolyte solution may additionally comprise, in addition to the conductive salts(s) and the solvent system, one or more additives for the purpose of improving electrical cycling performance. Suitable additives include, for example, unsaturated cyclic carbonates and unsaturated cyclic sulfonates, such as vinylene carbonate and substituted vinylene carbonates. The electrolyte solution may contain, for example, up to 5% by weight of such additives. In one embodiment, from 1 to 4% by weight additive(s) (e.g., vinylene carbonate) is present in the electrolyte solution.

One aspect of the present invention pertains to electrolyte solutions for use in lithium ion batteries. Of particular interest for such end use application are electrolyte solutions comprising a solvent system comprised of, consisting essentially of, or consisting of ethylene carbonate (EC) and methyl-(2-methoxyethyl)-carbonate (MMC). Such solvent systems exhibit a combination of desirable properties, including fire resistance, a low freezing point as well as good cycling conductive performance. Ethylene carbonate by itself is high boiling and thus fire resistant, but has a high melting point (36°C) which interferes with low temperature cycling performance due to freezing of the electrolyte. We have discovered that blending MMC with EC effectively lowers the melting point of the electrolyte solution, while not degrading the electrolyte performance. The presence of the MMC allows low temperature performance in a battery down to -20°C in the correct eutectic ratios, but the blend still has an acceptably high flash point. This desirable combination of attributes was unexpected, since adding other organic components such as propylene carbonate and buyrolactone to EC result in poorer cycling conductive performance as compared to EC alone and combining low boiling carbonates such as dimethyl carbonate and ethyl methyl carbonate with EC adversely affects the fire resistance of the electrolyte solution.

Such an EC/MMC solvent system may contain 10 to 80% by weight EC, with the balance being MMC. Optionally, the solvent system may additionally contain up to 5% by weight of a solvent other than EC or MMC, such as a cyclic or linear carbonate. Generally speaking, increasing the proportion of MMC present in the solvent system will lower the freezing point as well as the flash point, as compared to pure EC. Pure EC has a flash point of 151 °C as measured by a closed cup ignition test. A blend of 1/3 by weight MMC and 2/3 by weight EC will have a flash point, as measured by a closed cup ignition test, of 1 10°C and a freezing point of about -15°C. In one embodiment of the invention, an amount of EC is present in combination with MMC to provide a solvent system having a flash point of at least 65°C, at least 75°C or at least 80°C, as measured by a closed cup ignition test. In another embodiment of the invention, an amount of EC is present in the solvent system which is effective to prevent a sample of the solvent system held in an open container from igniting when an open flame is briefly contacted with the sample.

In other embodiments, an amount of MMC is present in combination with EC which is effective to provide a solvent system having a freezing point of -20°C or lower, -30°C or lower, -40°C or lower or -50°C or lower.

The conductive salt present in such an electrolyte solution (i.e., a solution comprised of an EC/MMC solvent system) is a lithium salt or combination of lithium salts, typically at a concentration of 0.5 to 2.0 M. Suitable lithium salts include LiPF₆, lithium triflate, LiBF₄ and the like and combinations thereof, including the additional lithium salts mentioned earlier herein.

Preparation of Novel Carbonate Solvents

A direct synthesis for the symmetrical and asymmetrical members of the family of organic carbonates is the following equilibration:

RO(C=0) OR¹ + R OM <→ RO(C=0)OR^" + R"0(C=0)OR' + R"0(C=0)OR" + R'OH + ROH

(in the presence of basic catalyst such as sodium methoxide)

In the case of dimethyl carbonate and 2-methoxy ethanol this becomes:

CH₃0(C=0)OCH₃ (DMC) + CH₃OCH₂CH₂OH ^ CH₃0(C=0)OCH₂CH₂OCH₃(MMC) +

CH₃OCH₂CH₂0(C=0)OCH₂CH₂OCH₃ (BMC) + CH₃OH

After equilibration, the catalyst can be neutralized with an acid or acid salt and the reaction mixture distilled to remove the alcohols and obtain pure organic carbonate components or a useful mixture of organic carbonates. Using an excess of the DMC in this case favors more of the desired MMC (asymmetrical component) as opposed to the symmetrical component bis-2-methoxyethyl carbonate (BMC). Mixtures of both with the starting material can be used as long as the alcohols are removed. The MMC is a preferred solvent of these series of novel organic carbonates and is new and novel.

Thus, one aspect of the invention provides a process for preparing a symmetrical or asymmetrical carbonate of the general formula:

RO(C=0)OR¹

R"OCH₂CH₂- and R"-0-(CH₂)_n-, n is 2, 3 or 4, R¹ is selected from the group consisting of CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-,

CH₃CH₂OCH₂CH₂- and R -0-CH₂CH₂-, and R" is methyl, ethyl, propyl or isopropyl, wherein the method comprises reacting a compound of the general formula RO(C=0)OR with a compound of the general formula R^!OH in the presence of a basic catalyst (e.g., sodium methoxide). For example, dimethyl carbonate may be reacted with 2-methoxyethanol to produce methyl-(2-methoxyethyl)carbonate, bis(2-methoxyethyl carbonate, or a mixture thereof.

According to one embodiment of the invention, MMC may be conveniently synthesized using the following method, which comprises the steps of: a) , reacting 2-methoxy ethanol and dimethyl carbonate in the presence of sodium methoxide to obtain an equilibration reaction mixture comprised of methanol, methyl- (2-methoxyethyl)-carbonate and bis-2-methoxyethyl carbonate;

b) . removing methanol from the equilibration reaction mixture by distillation;

c). neutralizing the equilibration reaction mixture from which methanol has been removed with an acid or acid salt to obtain a neutralized reaction mixture; and d). removing methyl-(2-methoxyethyl)-carbonate from the neutralized reaction mixture by distillation.

A molar excess of dimethyl carbonate may be used in step a) to favor the production of MMC. For example, the molar ratio of DMC to 2-methoxy ethanol may be at least 2: 1 , at least 3 : 1 , at least 4: 1 , or at least 5: 1. The initial equilibration may be carried out, for example, at a temperature of from about 20°C to about 70°C for a time of from about 30 minutes to about 3 hours, with the temperature of the reaction mixture being slowly increased during this time. Once the reaction mixture has equilibrated, the equilibration reaction mixture may be subjected to fractional distillation to remove substantially all or preferably all of the methanol present. The equilibration reaction mixture may then be treated with an amount of an acid or acid salt, such as sodium hydrogen phosphate, effective to neutralize any base present. Any insoluble products formed as a result of this neutralization step may be removed (by filtration, for example) from the neutralized equilibration reaction mixture prior to subjecting the neutralized equilibration reaction mixture to a subsequent distillation step in which purified MMC is recovered. Such distillation may be carried out as a fractional distillation using conventional techniques, wherein any remaining amount of DMC is removed overhead before then distilling over essentially pure MMC. Once this distillation step has been completed, the residual pot bottoms (i.e., the portion of the neutralized equilibration reaction mixture remaining after separation of the DMC and MMC) may be recycled for use in preparing a further batch of equilibration reaction mixture following the above-described procedure.

The practice of the present invention will employ, unless otherwise indicated, conventional techniques of synthetic chemistry, electrochemistry and battery/capacitor engineering that are within the skill of the art. Such techniques are explained fully in the literature. See, for example, March's Advanced Organic Chemistry, House's Modern

Synthetic Chemistry, Houben-Weyl's Methoden der organischen Chemie, Hier's text Organic Synthesis, U.S. Pat. No. 4,892,944 to Mori et al., and Lindens' Handbook of Batteries. It is to be understood that this invention is not limited to the particular cations or salts, methods of synthesis, solvents or the like, which are described in the preferred embodiments, as such may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

EXAMPLE 1

Preparation of methyl-(2-methoxyethyl)-carbonate (MMC) In a 2L 3-necked flask, 1749 g (19.41 moles) of dimethyl carbonate (DMC), 246.14 g (3.24 moles) of 2-methoxy ethanol, and 49.23 g (5 wt. %) of a 25% sodium methoxide in methanol solution were added. A 250°C thermometer was added to the flask and the reaction flask was then fitted to a distillation apparatus containing a thermometer and a collection flask. This was stirred at room temperature for 30 minutes until a cloudy, white suspension began to form. This mixture was then heated slowly to a reaction flask temperature of 70°C. At this point, the first drop of distillate was collected at 43 °C. The distillate was continuously collected and checked by gas chromatograph for the presence of products methyl-(2- methoxyethyl) carbonate (MMC) and bis-2-methoxy ethyl carbonate (BMC). This was continued until the reaction flask reached a temperature of 100°C and the distillation temperature reached 72°C. At this point, the reaction flask contents contained only a small percentage of DMC and no methanol. The reaction mixture was allowed to cool to room temperature. The reaction mixture was treated with NaH₂P0₄, mixed, and filtered. This step was then repeated with the filtrate to assure that no catalyst remained. The filtrate was then transferred to a 500 mL 3-necked flask and stirred. This was then vacuum-distilled slowly. The first fraction started at a distillation temperature of 28°C and a reaction flask temperature of 35°C. These fractions were mostly DMC. The MMC fractions were collected at a distillation temperature of 43°C under vacuum of 24 mm Hg and a reaction flask temperature of 55°C. The gas chromatograph showed at 91% yield of MMC of essentially 100% purity in the fraction of MMC product collected. The remainder of the mix was composed of 6% MMC and 9% BMC.

EXAMPLE 2

Preparation of Electrolyte Solutions

A. An electrolyte, diethyl dimethyl ammonium tetrafluoroborate, was dissolved in the mixed solvents of Example 1 to have a concentration of 2 M. B. An electrolyte, trimethyl ethyl ammonium tetralluoroborate, was dissolved in a mixed solvent EC:DMC 50:50 to prepare an electrolyte solution having a concentration of 2 M.

Alternatively, the solvent can comprise 100% MMC or 5% BMC, 45% MMC, 25% EC and 25% PC (all percentages are % by weight).

The electrolyte solution of part B coagulated before -40°C. The electrolyte solution of part A exhibited low temperature characteristics without coagulating and good conductivity in a wide temperature range down to -60°C. The electrolyte of part A can be used in capacitors used in a wide range of industries from miniature electronic instruments to large automobiles.

C. An electrolyte was made by dissolving lithium tetrafluoroborate in MMC to 1 M. This electrolyte was used in a lithium ion battery cell and would undergo charge/discharge cycle at -30°C.

Claims

What is Claimed is:

1. An electrolyte solution for low temperature capacitors or lithium batteries comprising a solvent system which is comprised of 20-100% by weight in total of at least one symmetrical or asymmetrical carbonate of the general formula (I):

RO(C=0)OR¹ ( I )

wherein R is selected from the group consisting of methyl, ethyl, isopropyl, propyl, CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, R"OCH₂CH₂- and R"-0-(CH₂)_n-, n is 2, 3 or 4, R¹ is selected from the group consisting of CH₃OCH₂CH₂-, CH₃OCH₂CH₂OCH₂CH₂-,

CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂-, CH₃CH₂OCH₂CH₂- and R -0-CH₂CH₂-, and R" is methyl, ethyl, propyl or isopropyl, and up to 80% by weight in total of at least one solvent which is different from the symmetrical or asymmetrical carbonate of general formula (I), said electrolyte solution additionally comprising at least one conductive salt and having a liquidus range of about -65°C to about 171°C.

2. The electrolyte solution of claim 1, wherein the at least one conductive salt includes at least one quaternary ammonium compound.

3. The electrolyte solution of claim 2 wherein the at least one quaternary ammonium

compound is selected from the group consisting of tetraethyl ammonium

tetrafluoroborate, triethylmethylammonium tetrafluoroborate,

diethyldimethylammonium tetrafluoroborate, ethyltrimethylammonium

tetrafluoroborate, dimethylpyrrolidinium tetrafluoroborate, diethylpyrrolidinium tetrafluoroborate, ethylmethylpyrrolidinium tetrafluoroborate, spiro-(l,l ')- bipiperidium tetrafluoroborate, and combinations thereof.

4. The electrolyte solution of claim 2 wherein the at least one quaternary ammonium

compound is present in the electrolyte solution in a concentration of 0.5 to 3 M.

5. The electrolyte solution of claim 1 wherein the solvent system is comprised of 100%) of carbonate of formula I.

6. The electrolyte solution of claim 1 wherein the solvent system comprises at least one of methyl-(2-methoxyethyl)-carbonate or bis-2-methoxyethyl carbonate.

7. The electrolyte solution of claim 1 wherein the at least one solvent which is different from the symmetrical or asymmetrical carbonate of general formula (I) is selected from the group consisting of linear carbonates, cyclic carbonates, carboxylic esters, ethers and combinations thereof.

8. The electrolyte solution of claim 1 wherein the at least one solvent which is different from the symmetrical or asymmetrical carbonate of general formula (I) is selected from the group consisting of cyclic carbonates and combinations thereof.

9. The electrolyte solution of claim 1 wherein the at least one conductive salt includes a quaternary ammonium tetrafluoroborate.

10. The electrolyte solution of claim 1 wherein the at least one conductive salt includes a lithium salt.

11. The electrolyte solution of claim 1 wherein the solvent system is comprised of methyl- (2-methoxyethyl)-carbonate and ethylene carbonate.

12. The electrolyte solution of claim 1 wherein the solvent system is comprised of 20 to 90 % by weight methyl-(2-methoxyethyl)-carbonate with the balance to 100% being ethylene carbonate, subject to the proviso that optionally up to 5% in total of one or more carbonates other than methyl-(2-methoxyethyl)-carbonate and ethylene carbonate may be additionally present in the solvent system.

13. The electrolyte solution of claim 12 wherein the conductive salt is a lithium salt.

14. The electrolyte solution of claim 1 additionally comprising up to 5% by weight of at least one additive selected from the group consisting of unsaturated cyclic carbonates and unsaturated cyclic sulfonates.

15. The electrolyte solution of claim 1 , wherein the solvent system consists of ethylene carbonate, methyl-(2-methoxyethyl)-carbonate and, optionally, up to 5% by weight in total of one or more solvents other than ethylene carbonate and mefhyl-(2- methoxyethyl)-carbonate, wherein an amount of methyl-(2-methoxyethyl)-carbonate is present which is effective to provide the solvent system with a freezing point of -20°C or lower.

16. A capacitor having an electrolyte solution, wherein the electrolyte solution is an

electrolyte solution in accordance with claim 1.

17. A capacitor having an electrolyte solution, wherein the electrolyte solution is an

electrolyte solution in accordance with claim 6.

18. The capacitor of claim 1 1, wherein the capacitor is an ultra capacitor or a double layer capacitor.

19. A lithium battery having an electrolyte solution, wherein the electrolyte solution is an electrolyte solution in accordance with claim 1.

20. A lithium battery having an electrolyte solution, wherein the electrolyte solution is an electrolyte solution in accordance with claim 6.

21. A lithium battery having an electrolyte solution, wherein the electrolyte solution is an electrolyte solution in accordance with claim 13.

22. A method of making methyl-(2-methoxyethyl)-carbonate, comprising the steps of: a) , reacting 2-methoxy ethanol and dimethyl carbonate in the presence of sodium methoxide to obtain an equilibration reaction mixture comprised of methanol, methyl- (2-methoxyethyl)-carbonate and bis-2-methoxyethyl carbonate;

b) . removing methanol from the equilibration reaction mixture by distillation;

23. An electrolyte solution comprising a solvent system comprised of methyl-(2- methoxy ethyl) -carbonate, said electrolyte solution additionally comprising at least one conductive salt and having a flash point as measured by a closed cup ignition test of greater than 65°C.

24. An ultracapacitor electrolyte having a flash point as measured by a closed cup ignition test of greater than 65°C.