Rechargeable lithium electrochemical cell usable at low...

Details Rechargeable lithium electrochemical cell usable at low... Rechargeable lithium electrochemical cell usable at low...

1998-12-29

2002-06-04

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S326000, C429S330000, C429S332000, C429S334000

Reexamination Certificate

active

06399255

ABSTRACT:

The present invention relates to a rechargeable lithium electrochemical cell that is usable at low temperature.
A lithium electrochemical cell possesses an electrochemical stack including a positive electrode comprising electrochemically active material capable of inserting lithium into its structure (generally an oxide of a transition metal, usually lithiated), and a negative electrode that supplies the lithium ions. The electrodes are placed on either side of a separator membrane that is generally made of polyolefin. The electrochemical stack is impregnated in a non-aqueous electrolyte that is solid or liquid. The electrolyte contains a lithium salt dissolved in a mixture of organic solvents.
The low temperature operation of such cells has frequently been investigated. Attention has been given mainly to the composition of the electrolyte.
According to document JP-08 195 221, low temperature (−20° C.) conductivity is increased when the solvent is made up of ethylene carbonate, propylene carbonate, an acetic ester, and at least one compound selected from diethyl or dimethyl carbonate, and dimethoxyethane. The volume fraction of the acetic ester is not more than 50%.
A rechargeable lithium electrochemical cell including an anode of metallic lithium or of lithium alloy, and a cathode whose active material is an electrically-conductive organic polymer, is described in document FR-2 641 130. The electrolyte is a lithium salt dissolved in a solvent, which salt is a combination of a cyclic carbonate and a non-cyclic carbonate. That cell retains sufficient discharge capacity at a temperature of less than 0° C.
U.S. Pat. No. 4,056,663 describes a rechargeable electrochemical cell having a metallic lithium anode and a metal oxide cathode, in which the electrolyte comprises a solvent that does not contain ether. The solvent is a mixture of carbonates or a mixture of at least one carbonate and at least one ester.
According to U.S. Pat. No. 4,983,476, improved low temperature performance is obtained by replacing the metallic lithium of the anode with a transition metal sulfide. The electrolyte comprises an aprotic organic solvent such as methyl formate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, tetrahydrofuran, and mixtures thereof.
The most recent rechargeable lithium electrochemical cells possess a negative electrode of the paste type having a conductive support that acts as a current collector on which there is placed a paste containing a binder and an electrochemically active material which is a material that is capable of inserting lithium into its structure. The greater safety of such cells makes them suitable for a wider range of applications. For use in particular in an electric vehicle or in radiocommunications, such cells must be capable of operating at low temperature, and in particular at below −20° C.
To this end, document EP-0 482 287 suggests using a rechargeable lithium electrochemical cell possessing a compressed anode containing carbon, a cathode comprising a lithiated oxide, and an electrolyte including a lithium salt dissolved in an organic solvent comprising a cyclic ester and a linear ester.
A rechargeable electrochemical cell having a carbon anode as described in document JP-09 022 738 contains an electrolyte whose solvent comprises a cyclic carbonate, a linear carbonate, and up to 44% by volume ethyl acetate. The binder of the electrodes is polyvinylidene fluoride, and the active material of the cathode is a lithium cobalt oxide. That cell has improved performance at low temperature. The document mentions a drop in performance at low temperature when methyl propionate or ethyl propionate is used as the solvent.
The discharged capacity of a rechargeable lithium electrochemical cell in accordance with document EP-0-531 617 can be increased. The cell comprises a carbon anode and a lithiated oxide cathode. The electrolyte solvent is a mixture of a cyclic carbonate, a linear carbonate, and a compound of formula RCOOR1, where R is an ethyl radical and R1 is an alkyl group having 1 to 3 carbon atoms. The respective volume fractions thereof are preferably 20% to 50%, 10% to 70%, and 10% to 70%.
Document EP-0 614 240 describes a rechargeable lithium electro-chemical cell having a carbon anode and a metal oxide cathode, with improved discharge at a high discharge rate, particularly at low temperature. The cell contains an electrolyte comprising a lithium salt and a mixture of aprotic solvents made up by volume of 10% to 20% ethylene carbonate, 5% to 40% propylene carbonate, and 50% to 85% dimethyl carbonate.
To improve high-rate discharge at low temperature, document EP-0 766 332 proposes an electrochemical cell comprising paste electrodes in which the binder is polyvinylidene fluoride (PVDF). It has an anode comprising an electrochemically active material based on carbon, and a cobalt oxide cathode. The solvent of the electrolyte is, by volume, made up of 50% to 60% of a mixture of cyclic carbonate and of cyclic ester, such as &ggr;-butyrolactone or &ggr;-valerolactone, 20% to 40% of a linear carbonate, and 10% to 25% of a linear ester.
Self-discharge during storage at low temperature is decreased in a rechargeable lithium electrochemical cell having a carbon anode, and a lithiated oxide cathode, in accordance with EP-0 548 449. The electrolyte solvent is a mixture of three components which are an aliphatic carboxylate, a cyclic carbonate, and a linear carbonate. The respective volume proportions thereof are preferably 10% to 80%, 20% to 50%, and not more than 70%.
An object of the present invention is to provide a rechargeable lithium electrochemical cell having a carbon anode in which performance during low temperature operation is better than that of known cells.
The present invention provides a rechargeable lithium electrochemical cell comprising an electrolyte containing a lithium salt dissolved in a non-aqueous solvent, at least one positive electrode, and at least one negative electrode of the paste type containing an electrochemically active material which is a carbon compound suitable for inserting lithium ions and a binder. The invention is characterized in that said solvent contains at least one saturated cyclic carbonate and at least one linear ester of a saturated aliphatic monocarboxylic acid, and in that said binder is a polymer having no fluorine.
The terms “linear ester of a saturated aliphatic monocarboxylic acid” and “saturated aliphatic carboxylate” are used to mean a compound of formula RC—O—OR′ in which R is H or an alkyl group, and R′ is an alkyl group such as CH
3
(methyl), CH
3
—CH
2
(ethyl), etc. . . . Said linear ester of a saturated aliphatic monocarboxylic acid is, for example, a formiate if R is H, an acetate if R is CH
3
, a propionate if R is CH
3
—CH
2
, a butyrate if R is CH
3
—(CH
2
)
2
, a valeriate if R is CH
3
—(CH
2
)
3
, etc. . . . .
In the solvent, the volume proportion of said saturated cyclic carbonate lies in the range 5% to 60% of said solvent, and the volume proportion of said linear ester lies in the range 20% to 85% of said solvent, the proportion of said linear ester is preferably not less than 50% of said solvent.
Said saturated cyclic carbonate is selected from propylene carbonate, ethylene carbonate, butylene carbonate, and mixtures thereof.
In a first variant, said saturated cyclic carbonate is ethylene carbonate.
In a second variant, said saturated cyclic carbonate is propylene carbonate.
In a third variant, said saturated cyclic carbonate is a mixture of ethylene carbonate and of propylene carbonate.
Said linear ester is selected from an acetate, a butyrate, a propionate, and mixtures thereof. By way of example, it is possible to select an ethyl acetate, a methyl acetate, a propyl acetate, an ethyl butyrate, a methyl butyrate, a propyle butyrate, an ethyl propionate, a methyl propionate, a propyl propionate.
In a first variant, said linear ester is ethyl acetate.
In a second variant, said linear ester is methyl butyrate.
In another implementation of the inven

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