GB2202670A - High energy calls - Google Patents

Abstract

The present invention provides nonaqueous cells having a highly active metal anode, such as lithium, a solid cathode of Bi2O3, and a liquid organic electrolyte, the electrolyte consisting essentially of 3-methyl-2-oxazolidone in combination with a solute, such as a metal salt, and, preferably, a low viscosity co-solvent, such as dioxolane, which offer both good service capacity and reduced bulging during use.

Description

High Energy Cells The present invention relates to nonaqueous cells with highly active metal anodes, solid Bi203 cathodes, and having a liquid organic electrolyte.

The development of high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium and the like, and the efficient use of high energy density cathode materials, such as By 203 The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realise the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the investigation of nonaqueous electrolyte systems and particularly to nonaqueous organic electrolyte systems.

Nonaqueous organic electrolytes in the art are electrolytes composed of a solute (such as a salt or complex salt of Group I-A, Group II-A or Group III-A elements of the Perodic Table) dissolved in an appropriate nonaqueous solvents. Conventional solvents include propylene carbonate, ethylene carbonate and y-butyrolactone. The term Periodic Table, as used herein, refers to the Periodic Table of the Elements as set forth on the inside back cover of the Handbook of Chemistry and Physics, 63rd Edition, CRC Press, Inc., Boca Raton, Florida, USA, 1982-1983.

A multitude of so lutes is known and recommended for use, but the selection of a suitable solvent has been particularly troublesome, since many of the solvents used to prepare electrolytes of sufficient conductivity are reactive with the anodes of such systems. The search for suitable solvents has concentrated on aliphatic and aromatic nitrogen- and oxygen-containing compounds, with attention also being given to organic sulphur-, phosphorus- and arsenic-containing compounds.

Results have not been satisfactory, as many of the solvents investigated still cannot be used effectively with extremely high-energy-density cathode materials and are still somewhat corrosive to lithium anodes, preventing efficient performance over any length of time.

U.S. Patent 4,085,259 discloses an electrochemical cell employing a lithium anode, a Bi203 cathode and an electrolyte solution of a conductive salt dissolved in a mixture composed mainly of propylene carbonate and dimethoxyethane. Although this cell gives good service capacity, it generally bulges excessively on discharge.

Japanese Patent Application 52/12425 discloses a cell employing a Bi203 cathode, a lithium anode and an organic electrolyte of y-butyrolactone and tetrahydrofuran as solvents with Lilo4.

Japanese Patent Application 56/159067 discloses a cell with a cathode of a higher bismuth oxide such as By 205 U.S. Patents 3,871,916, 3,951,685 and 3,996,069 disclose cells utilising a highly active metal anode, a solid cathode such as (CFX)n, CuO, FeS2, Cho304, V205, Pb304, In2S3 or CoS2, and a liquid organic electrolyte based on 3-methyl-2-oxazolidone in conjunction with a low viscosity cosolvent and a selected solute.

While the theoretical energy, i.e. the electrical energy potentially available from a selected anode-cathode couple is relatively easy to calculate, a nonaqueous electrolyte permitting the actual energy produced by an assembled battery to approach the theoretical energy must be selected. The problem is that it is practically impossible to predict in advance how well, if at all, a nonaqueous electrolyte will function with a particular anode-cathode couple. Thus a cell must be considered as a unit having three parts cathode, anode and electrolyte. It is axiomatic that the parts of one cell are not necessarily predictably interchangeable with parts of another cell to produce a useful result.

In a first aspect of the present invention there is provided a high energy density, nonaqueous cell having a highly active metal anode, a solid Bi203 cathode and a liquid organic electrolyte, the electrolyte comprising 3-methyl-2-oxazolidone (3Me20x) and a conductive solute.

It is an advantage of cells according to the present invention that they exhibit minimal bulge during discharge while providing good service output. Thus, they may be effectively used in applications involving tight height tolerances, e.g. watches with printed circuit boards.

According to a preferred aspect of the present invention, there is provided a cell as defined above, further comprising at least one low viscosity co-solvent in combination with the electrolyte.

Incorporation of a low viscosity co-solvent serves to reduce the viscosity of 3Me20x, allowing it to be used in cells subject to higher energy drains.

"Highly active" metal anodes suitable for use with the present invention include consumable metals, such as aluminium, the alkali metals, alkaline earth metals and alloys of alkali metals or alkaline earth metals with each other and other metals. The term 'alloy' includes: mixtures; solid solutions, such as lithium-magnesium; and intermetallic compounds such as lithium monoaluminide. The preferred anode materials are aluminium, calcium and the alkali metals, such as lithium, sodium and potassium. The most preferred anode is lithium since, in addition to being a ductile, soft metal that can easily be assembled into a cell, it possesses the highest energy-to-weight ratio of the group of suitable anode metals.

The solid Bi 203 cathode may be made by any suitable method known to those skilled in the art. In a preferred embodiment, the cathode material is mixed with 5 to 10% by weight of a conductive additive, such as carbon black, and with 2 to 10% by weight of a resin binder, such as polytetrafluoroethylene powder, and is then pressed into a finished cathode structure typically having from 7 to 20% by weight of combined conductive carbon and binder. U.S. Patent 4,085,259 describes suitable solid Bi203 cathodes.

In cells employing a cathode of Bi 203 there may be small proportions of higher oxides that are conductive during discharge and that give an undesirable initial higher discharge potential. These higher oxides in the By2 0 3 cathode can be reduced by using a current of about 2 mA cm 2 over a period of about five hours. U.S. Patent 4,085,259 describes mixing 5 to 30% by weight Sb2 03 with By203~ or mixing additional finely divided metallic bismuth with Bi203, to reduce the higher oxides.In the latter approach, the Bi 203 mass can be stirred with 0.5 to 5% by weight of bismuth in powder form, with grain size less than 60 Bm, and the resulting material can then be heated in an inert gas atmosphere at about 6000C for about 1 hour.

The higher oxides of bismuth may also be reduced by the method of U.S. Patent 4,163,829 in which a metallic reducing agent, such as zinc, is added to the cathode in an amount sufficient to reduce the higher oxides of bismuth in the cathode.

3-Methyl-2-oxazolidone (3Me20x), #H -CH -0-CO-#-CH 2 2 3' has a high dielectric constant, chemical inertness to battery components, a wide liquid range and low toxicity. However, when metal salts are dissolved in liquid 3Me20x for the purpose of improving conductivity, the viscosity of the solution becomes too high for efficient use as an electrolyte for nonaqueous cells, other than those requiring very low current drains.

Thus, the addition of a low viscosity co-solvent is necessary when 3Me20x is to be used as an electrolyte for nonaqueous cells designed to operate or perform at high energy density levels.

Low viscosity co-solvents suitable for use in accordance with the present invention include tetrahydrofuran (THF), dioxolane, dimethoxyethane (DME), dimethyl isoxazole (DMI), diethyl carbonate (DEC), ethylene glycol sulphite (EGS), dioxane, dimethyl sulphite (DMS), or the like. Tetrahydrofuran and dioxolane are preferred for their compatibility with metal salts dissolved in liquid 3Me20x and their chemical inertness to cell components. The total amount of the low viscosity co-solvent added is preferably between about 20% and about 80% based on total solvent volume (i.e. exclusive of solute) in order to lower the viscosity to a level suitable for use in a cell.

The use of small quantities of dimethyl isoxazole has been found to be advantageous when incorporated into the electrolyte. 'Small quantities' are generally less than about 2%, and an amount of about 0.2% is preferred.

Suitable conductive solutes (metal salts) for use in accordance with the present invention may be selected from the group MCF3S03, MSCN, MBF4, MClO4 and MM'F6 wherein M is lithium, sodium or potassium and M' is phosphorus, arsenic or antimony. The addition of solute improves the conductivity of 3Me20x, allowing it be used as an electrolyte in nonaqueous cell applications. The particular salt selected should be compatible and non-reactive with 3Me20x and the electrodes of the cell. The amount added should be sufficient to provide good conductivity, e.g. at least about ohm#1 -l about 10 ohm cm . For most cell applications, an amount of at least about 0.5 M is sufficient.

The present invention is further illustrated by the following Examples.

EXAMPLE 1 The viscosity of several samples of 3Me20x, with and without a conductive solute and/or a low viscosity co-solvent, were obtained using a Cannon-Fenske viscometer. The data obtained are shown in Table 1 and demonstrate the high viscosity of a solution of 3Me20x containing a dissolved conductive solute. As shown by sample 2, when one mole of LiClO4 is added to one litre of 3Me20x, the viscosity of the solution is in the region of 6.61 centistokes. When one mole of the same metal salt, LiClo4, is added to one litre of equal parts of 3Me20x and tetrahydrofuran (Sample 6), the viscosity of the solution is only about 2.87 centistokes. This shows that the viscosity of a solution of 3Me20x and a metal salt is advantageously decreased by the addition of a specifically selected, low viscosity co-solvent.

TABLE 1 Viscosity SamPle Solvent and Salt (Centistokes) 1 3Me20x; no salt 2.16 2 3Me20x; 1M Liy104 6.61 3 3Me20x; 1M LiBr 7.58 4 50-50 3Me20x, THF; no salt 1.05 5 50-50 3Me20x, THF; 1M LiAsF6 3.59 6 50-50 3Me20x, THF; 1M Liy104 2.87 7 25-75 3Me20x, THF; 1M LiAsF6 2.08 8 25-75 3Me20x, dioxolane; 1M LiAsF6 1.83 9 25-75 3Me20x, THF; 1M Liy104 1.99 EXAMPLE 2 Miniature cells (measuring 0.374 inch (9.50mm) in diameter and 0.105 inch (2.67mm) high) were constructed each using a cathode of Bi203, a lithium anode and an electrolyte of LiClO dissolved in a mixture 4 comprising propylene carbonate and dimethoxyethane.

Similar cells were constructed except that the electrolyte consisted of LiCF3 SO3 dissolved in a mixture of 30% dimethoxyethane, 30% 3Me20x, 40% 1,3-dioxolane and a trace (0.2%) of dimethyl isoxazole.

Each of the cells was discharged at 210C for one month on a 30K ohm background load while a 2-second pulse discharge was applied across a 2.4K load 12 times a day. At the end of the test, the heights of the cells were measured. The average height of 3 cells employing the PC-DME-LiCl04 electrolyte was found to have increased by 0.007 inch (0.178mm). The average height of 3 cells employing the 3Me20x electrolyte was found to have increased by only 0.002 inch (0.05mm).

The average service output of each cell lot tested was 9 mAh to a 1.2. volt cut-off. These data show that cells according to the invention deliver excellent service output while exhibiting minimal bulge.

ExamPle 3 Miniature button cells (measuring 0.455 inches (11.6mm) in diameter and 0.118 inch (3.00m) high) were constructed each using a cathode of Bi 203r a lithium anode and an electrolyte of Lilo4, dissolved in a mixture comprising propylene carbonate and dimethoxyethane. Similar cells were constructed except that the electrolyte consisted of LiCF3 SO3 dissolved in a mixture of 30% dimethoxyethane, 30% 3Me20x, 40% 1,3-dioxolane and a trace (0.28) of dimethyl isoxazole.

Each of the cells was discharged at 210C on a 15K ohm background load while a 2-second pulse discharge was applied across a 2.4K load 12 times a day. At the end of the test, the heights of the cells were measured.

The average height of 3 cells employing the PC-DME-LiC104 electrolyte was found to have increased by 0.012 inch (0.305mm). The average height of 3 cells employing the 3Me20x increased by only 0.003 inch (0.077mm).

The average service output of each cell lot tested was 76 mAh to a 1.2 volt cut-off for the cells employing the PC-DME-LiCl04 electrolyte, while the cells employing the 3Me20x electrolyte delivered a service output of 83 mAh. Again these data show that cells according to the invention deliver excellent service output while exhibiting minimal bulge.

ExamPle 4 Minature button cells (measuring 0.455 inch (11.6mm) in diameter and 0.118 inch (3.00mm) high) were constructed each using a cathode of Bi203, a lithium anode and an electrolyte of Liy104 dissolved in a mixture comprising propylene carbonate and dimethoxyethane. Similar cells were constructed, but using an electrolye consisting of LCF SO dissolved in a mixture of 30% dimethoxyethane, 30% 3Me20x, 40% 1,3-dioxolane and a trace (0.2%) of dimethyl isoxolane.

Each of the cells was discharged at 210C on a 200K ohm background load while a 2-second pulse discharge was applied across a 2K load once a day. At the end of the test, the heights of the cells were measured. The average height of 3 cells employing the PC-DME-LiC104 electrolyte was found to have increased by 0.010 inch (0.25mm). The average height of 3 cells employing 3Me20x was found to have increased by only 0.003 inch (0.077mm).

The average service output of the cell lot tested was 80 mAh to a 1.2 volt cut-off for the cells employing the PC-DME-LiCl04 electrolyte while the cells employing the 3Me20x electrolyte delivered a service output of 84 mAh. The cells according to the invention again delivered excellent service output and minimal bulge.

Claims (12)

1. A nonaqueous cell having a highly active metal anode, a solid By203 cathode and a liquid organic electrolyte, the electrolyte comprising 3-methyl-2oxazolidone and a conductive solute.

2. A nonaqueous cell according to claim 1 wherein the electrolyte further comprises at least one, low viscosity co-solvent.

3. A nonaqueous cell according to claim 2, wherein the co-solvent is tetrahydrofuran, dioxolane, dimethoxyethane, dimethyl isoxazole, diethyl carbonate, ethylene glycol sulphite, dioxane or dimethyl sulphite.

4. A nonaqueous cell according to any preceding claim, wherein the conductive solute is MCF3S03, MSCN, MBF4, MC104 or MM'F6, wherein M represents lithium, sodium or potassium and M' represents phosphorus, arsenic or antimony.

5. A nonaqueous cell according to any preceding claim, wherein the active metal anode is lithium, potassium, sodium, calcium, magnesium, or an alloy thereof.

6. A nonaqueous cell according to any preceding claim, wherein the co-solvent is a mixture of dimethoxyethane and dioxolane.

7. A nonaqueous cell according to any preceding claim, wherein the conductive solute is LiBF4, LiC104, or LiCF3S03.

8. A nonaqueous cell according to any preceding claim, wherein the anode is lithium.

9. A nonaqueous cell according to any preceding claim, wherein the co-solvent consists of a mixture of dimethoxyethane, 1,3-dioxolane and a small amount of dimethyl isoxazole.

10. A nonaqueous cell according to claim 9, wherein the co-solvent contains about 0.2% of dimethyl isoxazole.

11. A nonaqueous cell according to any preceding claim, wherein the co-solvent forms between about 20% and about 80%, v/v, of the electrolyte.

12. A nonaqueous cell according to claim 1, substantially as described herein, with particular reference to the accompanying Examples.