GB2054948A - Non-aqueous electrolyte - Google Patents

Abstract

There is disclosed a non-aqueous electrolyte comprising at least one solvent selected from the group consisting of sulfolane and its liquid alkyl substituted derivatives thereof, a cosolvent of the formula CH3O (CH2CH2O)nCH3 where n has the value defined in claim 1, and a solute selected from the group consisting of lithium tetrafluoroborate, lithium perchlorate and mixtures thereof. The electrolyte is useful in lithium cells and there is also disclosed a rechargeable cell comprising a lithium anode, a cathode and a non-aqueous electrolyte of the invention.

Description

SPECIFICATION Non-aqueous electrolyte A rechargeable nonaqueous Li/TiS2 cell employing a nonaqueous electrolyte comprising at least one solvent selected from the group consisting of sulfolane and its liquid alkyl-substituted derivatives, a cosolvent of the formula CH30(CH2CH20)nCH3 where n varies between 1 (dimethoxyethane) and 4 and an ionizable solute selected from the group consisting of lithium tetrafluoroborate, lithium perchlorate and mixtures thereof.

Lithium cells have the possibility of high energy density because of the low equivalent weight of the metal. As a result, several primary high energy density nonaqueous systems have been developed in the past few years. Secondary lithium cells, however, have been difficult to produce since many of the known solvents employed in the electrolyte solution foster dendritic deposits during the charging mode of operation which subsequently causes cell shorting. It is also known that the plated lithium is reactive toward the commonly used solvents and impurities contained therein, thus giving rise to substantial corrosion rates. These corrosion reactions may result in formation of isolated, electrochemically unusable lithium that, in some instances, may result in lithium deposits that separate from the substrate material.This would result in poor cyclability characteristics of the lithium anode.

It has been disclosed in the prior art that extensive purification of solvents and electrolytes are required to produce a solvent-electrolyte suitable for electrodeposition of lithium on a substrate.

However, even if lithium can be deposited on a substrate using a particular solvent-electrolyte it is not always true that the solvent-electrolyte can be used with a lithium-cathode couple to produce a rechargeable cell.

While the theoretical energy, i.e., the electrical energy potentially available from a selected anode cathode couple, is relatively easy to calculate, there is a need to choose a nonaqueous electrolyte for such a couple that permits the actual energy produced by an assembled battery to approach the theoretical energy. The problem usually encountered is that it is practically impossible to predict in advance how well, if at all, a nonaqueous electrolyte will function with a selected couple. Thus a cell must be considered as a unit having three parts; a cathode, an anode and an electrolyte, and it is to be understood that the parts of one cell are not predictably interchangeable with parts of another cell to produce an efficient and workable cell.

U.S. Application Serial No. 890,971 filed March 28, 1978 in the name D. V. Louzos et al, discloses a solvent-electrolyte use in a process for the electrodeposition of lithium comprising lithium fluoroborate dissolved in a mixture of methylene chloride and sulfolane and/or its alkyl-substituted derivatives thereof.

U.S. Patent 4,009,052 discloses a battery utilizing lithium as the anode-active material, titanium disulfide as the cathode-active material and lithium perchlorate dissolved in tetrahydrofuran plus dimethoxyethane solvent as the electrolyte.

U.S. Patent 3,907,597 discloses a nonaqueous cell utilizing a highly active metal anode such as lithium, a solid cathode such as fluorinated carbon, copper sulfide, copper oxide, manganese dioxide, lead dioxide, iron sulfide, copper chloride, silver chloride and sulfur, and a liquid organic electrolyte comprising sulfolane or its liquid alkyl-substituted derivatives in combination with a cosolvent such as dimethoxyethane and an ionizing solute such as lithium perchlorate and lithium tetrafluoroborate.

The invention relates to a rechargeable cell comprising a lithium anqde, a cathode such as a titanium disulfide cathode, and a nonaqueous electrolyte comprising at least one solvent selected from the group consisting of sulfolane and its liquid alkyl-substituted derivative thereof, a cosolvent of the formula CH30(CH2CH2O)nCH3 where n varies between 1 (dimethoxyethane) and 4, and a solute selected from the group consisting of lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4) and mixtures thereof.

Also according to the invention there is provided a non-aqueous electrolyte comprising at least one solvent selected from the goup consisting of sulfolane and its liquid alkyl substituted derivatives thereof, a cosolvent of the formula CH3O(CH2CH2O)#CH3 where n has the value defined in claim 1, and a solute selected from the group consisting of lithium tetrafluoroborate, lithium perchlorate and mixtures thereof.

Preferably the sulfolane and/or the alkyl-substituted derivatives thereof should consist of between about 20 and about 80 volume per cent of the electrolyte solvent mixture with the remainder being the cosolvent a#nd most preferably between about 50 and 80 volume percent of the electrolyte solvent mixture. The preferred cosolvent is dimethoxyethane.

It has been found that using the above electrolyte solution in a lithium cell, a rechargeable lithium cell is produced that does not require the tedious purification procedures that were usually necessary in the prior art in the production of rechargeable lithium cells. The rechargeable cells of this invention have been found to operate efficiently during numerous charge and discharge cycles without effectively producing dendritic deposits during the charging mode of operation. It has also been observed that using the electrolyte solution of this invention along with a lithium/titanium disulfide couple, lithium can be efficiently electrodeposited on the lithium electrode substrate during charging thus making this cell system an excellent rechargeable lithium cell.

Sulfolane for use in this invention is a 1,1 -dioxotetrahydrothiophene (sometimes called tetramethylene sulfone) and is a saturated heterocyclic compound of the structure:

Some of the physical properties of sulfolane are shown in Table 1: TABLE 1 Melting Point (OC) 28 Boiling Point (OC) 283 Sp. Cond., 250C (ohm~' cm-') 2 > c 10-8 Dielectric Constant, 250C 44 Density, 300C (g/cm3) 1.2615 Viscosity, 300C (centipoise) 9.87 Freezing Point Depression Constant 66.2 The 3-methyl sulfolane, which is a liquid alkyl-substituted derivative of the above structure and is also suitable for use in this invention, has the following structure::

Sulfolane and its liquid alkyl-substituted derivatives, such as 3-methyl sulfolane, are good nonaqueous solvents but have the disadvantage in that they have a relatively high viscosity. Thus when metal salts are dissolved in these solvents for the purpose of improving the conductivity of the solvents, the viscosity of the solution becomes too high for its efficient use as an electrolyte for nonaqueous cell applications. Consequently the addition of a low viscosity cosolvent is necessary if sulfolane and its liquid alkyl-substituted derivatives are to be used as an electrolyte for nonaqueous cells which can operate or perform at a high energy density level.

Although many cosolvents and metal salts are disclosed in the prior art, it has been found that when a cosolvent, such as dimethoxyethane, is used along with lithium tetrafluoroborate, lithium perchlorate or mixtures thereof in conjunction with sulfolane andlbr its liquid alkyl-substituted derivative, a solvent-electrolyte is produced that is admirably suited for use in the electrodeposition of lithium. This discovery makes this solvent-electrolyte ideally suited for use in rechargeable lithium cells using various cathodes.

Thus in accordance with the present invention, the electrolyte solvent mixture is preferably composed of from about 20 to about 80 volume per cent of sulfolane and/or the alkyl-substituted derivatives thereof, with the remainder being a cosolvent such as dimethoxyethane along with lithium tetrafluoroborate, lithium perchlorate or mixtures thereof substantially dissolved in said solvent mixture.

This solvent-electrolyte when used in a lithium cell will produce a coherent layer on nondendritic lithium deposited on the anode during the cell's charging mode of operation. When the concentration of the sulfolane and/or the alkyl-substituted derivatives thereof are below 20 volume per cent of the electrolyte solvent mixture, then using the electrolyte mixture in a rechargeable lithium cell will result in a slightly dendritic deposit of lithium on the anode during the cell's charging mode of operation. When the sulfolane and/or the alkyl-substituted derivatives thereof are present in a concentration of above 80 volume per cent, then the electrolyte would be too viscous for efficient high current drain applications.

The concentration of the metal salts lithium tetrafluroborate and/or lithium perchlorate can vary in the solvent although it has been found that a 1.5 molar concentration is preferable.

EXAMPLE I To study the effects of various electrolytes upon the morphology of lithium electrodeposits, glass test cells were constructed using two spaced-apart, essentially parallel lithium electrodes in about 20 to 30 ml of an electrolyte shown in Table 1. Each electrode was one-centimeter by 2-centimeters thereby providing two square centimeters of lithium area available on each side. A current density of 2 milliamperes per square centimeter was used to discharge (lithium stripping) and charge (lithium plating) the cells. Each cell was discharged for four hours, followed by being charged for four hours and this cycle was repeated for each cell a number of times as shown in Table 1. The adherent lithium plate, determined by use of the conventional hydrogen evolution test, was evaluated as per cent of the coulombically calculated deposit expressed as efficiency for both electrodes.The data so obtained, including the conductivity of each electrolyte, are shown in Table 1.

TABLE 1 Electrolyte Solution Efficiency ( /0) Conductivity Solvent Salt Anode Cathode Cycles (##cm)-lx 10-i DIOX 2.5-M Lilo4 47 47 5 8.8 DIOX 1.5-MLiClO4 42 56 5 5.1 *DIOX-SULF 1.5-M Lilo4 47 56 3 5.7 *DMESULF 1.5-M LiBF4 66 66 5 4.2 *DME-SULF 1.5-M Lilo4 68 66 5 6.6 *DME-SULF 9:1 LiBF4-LiClO4*** 68 81 4 4.6 **80DME-20SULF 1.5-M LiBF4 56 66 5 5.6 **70y-BL-30DME 1-M Lilo4 40 61 5 12.0 DIOX is dioxolane; SULF is sulfolane; y-BL is y#butyrnlactone.

DME is 1 ,2dimethoxyethane.

*equal volume of solvents.

**expressed in volume per cent.

***ratio LiBF4-LiCl04, 1.5M.

As shown in Table 1, the obtained adherent deposit from 2.5-M LiClO4-dioxolane (DIOX) electrolyte was only 47% of the calculated plate for the five cycles. Plating from 1 .5-M LiCl94-Dl0X- sulfolane (SULF) solution was somewhat improved on the electrode. Using the identical procedure and 70% y-butyrolactone (}7-BL)-30% 1 ,2-dimethoxyethane (DME) with 1-M UCIO, also gave an improvement of the plating on the electrode. The best plating efficiencies were obtained in DME-SULF cosolvent mixtures with either LiBF4 or Lilo4 salt or mixtures thereof.

EXAMPLE II Sealed cells were produced using a lithium anode, a titanium disulfide cathode and an electrolyte solution as shown in Table 2. The cells were tested as in Example I and the data obtained are shown in Table 2.

TABLE 2 Electrolyte Solution Efficiency (%) Solvent Salt Anode Cathode Cycles *DME-SULF 1.5-M LiBF4 85 95 5 92 95 15 94 96 20 **80DME~20SULF 1.5-M LiBF4 94 96 17 *equal volume of solvents.

**80 vol % DME~20 vol. % SULF.

EXAMPLE Ill A sealed cell was produced employing a lithium anode, a titanium disulfide cathode and an electrolyte solution consisting of 1.5 molar LiBF4 in 50 volume per cent 1 ,2-dimethoxyethane-50 volume per cent sulfolane. The cell was discharged at a current density of 2 milliamperes per square inch for 3 1/2 hours and then charged at 0.5 milliampere per square inch for 16 hours. This discharge charge cycle was continued for 1 26 times and the total output for the cell was calculated to be 885 milliampere-hours. These results show that more than three times the primary capacity of the lithium anode had been delivered and more than 27 times the primary capacity of the titanium disulfide cathode at a voltage above 1.6 volts.

It is to be understood that other modifications and charges to the preferred embodiments of this invention herein shown and described can also be made without departing from the spirit and scope of the invention.

Claims (17)

1. A rechargeable cell comprising a lithium anode, a cathode and a nonaqueous electrolyte comprising at least one solvent selected from the group consisting of sulfolane and its liquid alkylsubstituted derivatives thereof, a cosolvent of the formula CH3(CH2CH2O) #CH3 where n varies between 1 and 4, and a solute selected from the group consisting of lithium tetrafluoroborate, lithium perchlorate and mixtures thereof.

2. A rechargeable cell as claimed in claim 1, wherein the cosolvent is 1 ,2-dimethoxyethane.

3. A rechargeable cell as claimed in claim 1 or 2, wherein the cathode is titanium disulfide.

4. A rechargeable cell as claimed in any one of the preceding claims, wherein the at least one solvent selected from the group consisting of sulfolane and its alkyl-substituted derivatives thereof is between about 20 and about 80 volume per cent of the electrolyte solvent mixture.

5. A rechargeable cell as claimed in any one of the preceding claims, wherein the at least one solvent selected from the group consisting of sulfolane and its alkyl-substituted derivatives thereof is between about 50 and about 80 volume per cent of the electrolyte solvent mixture.

6. A rechargeable cell as claimed in claim 1, wherein the cathode is titanium disulfide, the solvent is sulfolane, the cosolvent is 1,2-dimethoxyethane and wherein the sulfolane is between about 20 and about 80 volume per cent of the electrolyte solvent mixture.

7. A rechargeable cell as claimed in any of the preceding claims, wherein the solute is lithium tetrafluoroborate.

8. A rechargeable cell as claimed in any one of claims 1 to 7, wherein the solute is lithium perchlorate.

9. A rechargeable cell as claimed in claim 1, substantially as hereinbefore described in any one of the foregoing Examples.

10. A non-aqueous electrolyte comprising at least one solvent selected from the group consisting of sulfolane and its liquid alkyl-substituted derivatives thereof, a cosolvent of the formula CH3O(CH2CH2O)nCH3 where n has the value defined in claim 1, and a solute selected from the group consisting of lithium tetrafluoroborate, lithium perchlorate and mixtures thereof.

11. An electrolyte as claimed in claim 10, wherein the co-solvent is 1 ,2-dimethoxyethane.

12. An electrolyte as claimed in claim 10 or 11, wherein the at least one solvent comprises between about 20 and about 80 volume per cent of the electrolyte solvent mixture.

13. An electrolyte as claimed in any one of claims 10 to 12, wherein the at least solvent comprises between about 50 and about 80 volume per cent of the electrolyte solvent mixture.

14. An electrolyte as claimed in claim 10, wherein the solvent is sulfolane, the co-solvent is 1,2-dimethoxyethane and wherein the sulfolane comprises between about 20 and about 80 volume per cent of the electrolyte solvent mixture.

15. An electrolyte as claimed in any one of claims 10 to 14, wherein the solute is lithium tetrafluoroborate.

16. An electrolyte as claimed in any one of claims 10 to 14, wherein the solute is lithium perchlorate.

17. An electrolyte as claimed in claim 10 substantially as hereinbefore described in any one of the foregoing Examples.