Improved non-aqueous active metal cell.

Abstract: An improved non-aqueous electrolyte solution for lithium cells is disclosed which utilizes an organic ester of methyl formate together with lithium salt and an amount of CO₂ in the electrolyte solution. The active lithium anode may also be pretreated with CO₂.

Description: The present invention relates to non-aqueous electrochemical cells and, more particularly, to improvements with respect to the recharging of secondary active metal cells including those utilizing anodes of lithium metal. The invention allows such cells to achieve suitable high recharging efficiencies without sacrifice of high cell capacity. As is well known in the art, non-aqueous active metal cells have enabled those skilled in the art to achieve much higher energy densities or energy to weight ratios than was heretofore known. The potential uses for such cells has created a great deal of interest, resulting in significant efforts to improve the safety and versatility of these cells. In the book "Industrial Chemistry" (Chapman and Hall Ltd. 1982) D. Pletcher refers on page 271 to different types of ambient-temperature lithium cells which have in common the dissolution of lithium as the negative electrode. One type of electrolyte is an aprotic organic solvent containing a lithium salt. The organic solvent could include methyl formate, while popular electrolytes are inter alia hexafluoroarsenate. From JP 59 128 760 (Chemical Abstract 102: 35345t) a non-aqueous electrolyte battery is known wherein a protective film is formed on a lithium anode before the anode is incorporated in the battery. Lithium is formed into a ribbon-like shape in dry air by extrusion molding, sealed and stored in Ar, punched into disks in dry air, pressed and integrated with an anode current collector net, then left in a CO₂ atmosphere before adding electrolyte in the battery fabrication. Such cells typically consist of a light, strongly reducing anode, normally an alkali metal such as lithium, an aprotic, non-aqueous solvent into which has been dissolved an appropriate quantity of a salt of the anode metal to form a conductive solution, and an oxidizing agent as the cathode material. In some instances, the electrolyte solvent or co-solvent also acts as the active cathode material which is subsequently reduced at a porous carbon electrode. Such dual-function solvents include thionyl chloride, sulfuryl chloride, and sulfur dioxide. While primary lithium cells have been in use for quite some time, the development of a practical rechargeable lithium cell has been severely hindered by the inability to efficiently recharge the lithium anode. Because of its high reactivity, a significant fraction of the lithium metal plated out during recharging undergoes side reactions with the electrolyte solution and thereby becomes sheathed in a layer of nonconductive reaction products. This encapsulated lithium is electrically insulated from the anode and thus can no longer participate in the electrochemical reactions. Some lithium is lost in this manner during each recharge cycle until virtually the whole anode is consumed. The lithium cycling efficiency provides a measure of the rechargeability of the lithium anode in a given electrolyte solution and values of greater than 90%, and preferably greater than 95%, are needed if rechargeable lithium cells are to be practical. In the prior art, most rechargeable lithium cells employed an organic solvent-based electrolyte solution with an insoluble, solid oxidant cathode material. Ether-based electrolyte solutions have been predominately used in these cells. In particular, cells utilizing LiAsF₆ dissolved in 2-methyl tetrahydrofuran (2-methyl THF) as the electrolyte solution have provided the best overall performance, yielding lithium cycling efficiencies of up to 97% in prototype hardware cells. However, while these solutions are very promising with respect to lithium cycling efficiency, they exhibit low conductivities which severely degrade cell performance at low temperatures (i.e. below 0°C) or at high rates of discharge. There remains a need in the art for secondary rechargeable cells exhibiting both of the desirable characteristics including: 1. High lithium cycling efficiencies so that a practical number of cycles can be achieved during the life of the cell and, 2. sufficient cell conductivity so that the advantage obtained by the high energy density nature of the cell can be maintained over a broad temperature interval and over a wide range of discharge rates. Thus, the main object of the present invention is to improve the efficiency of secondary cells while a further object is to improve the efficiency of primary cells. These and other objects are achieved by a non-aqueous cell as characterized in claim 1. SUMMARY OF THE INVENTION The present invention concerns significant improvements in secondary active metal, non-aqueous cells having organic-based electrolyte solutions, particularly solutions which employ ester solvents. It enables the two goals mentioned above to be reached. According to the present invention, it has been discovered that carbon dioxide (CO₂) has highly beneficial effects in enhancing desirable properties of such secondary cells. The carbon dioxide can be used in two ways, namely, to pretreat the lithium anode prior to exposure to the electroyte solution and as an electrolyte solution additive. The use of carbon dioxide as an electrolyte solution additive, however, has been found to produce the greatest effect. In the preferred embodiment, the anode is lithium and the active cathode material is a transition metal oxide such as V₂O₅. The preferred electrolyte ester solvent is methyl formate (HCOOCH₃). The electrolyte salt normally used is LiAsF₆ and an amount of LiBF₄ may also be present. Ester-based solutions can offer conductivities almost an order of magnitude greater than prior art solutions, thus allowing cells to operate efficiently over the entire temperature range from -40°C to +75°C. The CO₂-doped electrolyte solutions of the present invention are used principally with respect to secondary or rechargeable cells. However, it has also been discovered that CO₂ reduces anode polarization and appears to reduce passivation effects in methyl formate solutions, thus offering benefits for primary cell applications as well. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIGURE 1 compares the specific conductivity of several electrolyte solutions over a wide temperature range; FIGURES 2 and 3 show the specific conductivity at 25°C versus electrolyte concentration, without and with LiBF₄, in methyl formate; FIGURE 4 shows lithium cycling efficiency versus electrolyte concentration, with and without CO₂, in methyl formate. DESCRIPTION OF THE PREFERRED EMBODIMENTS The two most important factors concerning electrolyte solutions for secondary or rechargeable cells of the class described are (1) maintaining sufficiently high conductivities to allow cells to operate over a broad temperature range at moderate to high rates of discharge and (2) providing for efficient cycling of the lithium anode such that a sufficient number of recharging cycles are possible. FIGURE 1 illustrates the dramatic increase in specific conductivity that can be realized with ester-based electrolyte solutions, exemplified by the methyl formate solution, over current state-of-the-art solutions such as the 1.5M LiAsF₆/2-methyl THF solution. This superiority of the ester-based solutions holds throughout the full military specification temperature range of -40°C to +75°C. In fact, the methyl formate solutions are better by almost an order of magnitude over most of this range. LiBF₄ is normally used in methyl formate solutions to improve solution stability at elevated temperatures and is generally added at one fifth of the LiAsF₆ concentration. FIGURES 2 and 3 show that the specific conductivity of the methyl formate solutions reach a maximum at a LiAsF₆ concentration between 1.5 and 2M and that the added LiBF₄ has very little effect on the specific conductivity. The data given in TABLE I further show that methyl formate solutions offer excellent conductivities over a wide range of solute concentrations. In fact, the value of 17.3 mMHO/cm yielded by a 0.5M LiAsF₆ solution is even sufficient for most cell applications. CONDUCTIVITY VS SOLUTE CONCENTRATION XM LiAsF₆ + X/5M LiBF₄/MF ELECTROLYTE SOLUTIONS LiAsF₆ CONCENTRATION, mol/L moles per liter LiBF₄ CONCENTRATION, mol/L moles per liter SPECIFIC CONDUCTIVITY, mMHO/CM 0.5 0.1 17.3 1.0 0.2 34.4 1.5 0.3 43.0 2.0 0.4 43.1 2.5 0.5 37.3 3.0 0.6 28.2 Temperature = 25 °C The second key factor for any electrolyte solution considered for use in a rechargeable lithium cell is to provide for efficient cycling of the lithium anode. To achieve long cycle life capabilities in practical hardware, lithium cycling efficiencies of greater than 90% and preferably greater than 95% are required. Lithium cycling efficiency tests were conducted by stripping and plating lithium on a working lithium electrode at 1.0 mA/cm². The charge passed during each half-cycle represented approximately a 20 percent depth of discharge, based on the starting capacity of the working lithium electrode. The tests were continued until the working lithium electrode was completely consumed. The cycling efficiency was then determined using the following relationship: where, E = Lithium cycling efficiency, % Co = Starting capacity of lithium working electrode, mAh Ci = Capacity of lithium plated and stripped during each half-cycle, mAh n = Total number of cycles achieved With respect to the use of CO₂, pretreatment of the working lithium electrodes was carried out by storing the electrodes in a sealed vessel with 2.5 Kg/sq.cm (= 20 psig) CO₂ pressure for 3, 24, or 120 hours. This technique was found to yield the greatest improvements in the more concentrated solutions. For example, the measured lithium cycling efficiency for a 2M LiAsF₆ + 0.4M LiBF₄/MF solution was increased from 70% to 81% while no change in efficiency was observed for a 0.5M LiAsF₆ + 0.1M LiBF₄/MF solution. The highest lithium cycling efficiency obtained using this approach was 87 percent for a 1.0M LiAsF₆ + 0.2M LiBF₄/MF solution. In the tests involving CO₂ as an electrolyte additive, solutions were normally first saturated with CO₂ and then, following activation, the cells were pressurized with CO₂ gas (pressures of 1.6 Kg/sq.cm and 4.6 Kg/sq.cm (= 8 and 50 psig)were employed in the tests conducted). Table II compares the measured efficiency values of methyl formate solutions with and without CO₂ added while Figure 4 shows the results graphically. As can be seen from the results, adding CO₂ to the solutions brought about a dramatic improvement in lithium cycling efficiency. In fact, the efficiency value of 98% achieved with the 1.0M LiAsF₆ + 0.2M LiBF₄/MF solution exceeds the best reported value for 2-methyl THF solutions. Furthermore, CO₂ was also observed to decrease anode polarization levels and appears to reduce anode passivation effects in the methyl formate solutions. Significantly, these enhancements were all achieved without any loss in conductivity or other detrimental effects. LITHIUM CYCLING EFFICIENCIES FOR XM LiAsF₆ + X / 5 M LiBF₄ /MF ELECTROLYTE SOLUTIONS LiAsF₆ CONCENTRATION, LiBF₄ CONCENTRATION, LITHIUM CYCLING EFFICIENCY, COMMENTS 0.5 0.1 84.6 Without CO₂ 1.0 0.2 81.1 Without CO₂ 2.0 0.4 69.7 Without CO₂ 0.5 0.1 96.3 With CO₂ 1.0 0.2 98.3 With CO₂ 2.0 0.4 93.0 With CO₂ NOTES: 1) All tests were conducted at ambient temperature using a stripping/plating current density of 1.0 mA/cm². 2) Carbon dioxide (CO₂) was added by pressurizing the test cells with 4.6 Kg/sq.cm (= 50 psig)of CO₂ gas. Although the mechanism is not well understood at this time, it is believed that CO₂ acts as a precursor to form an ionically conductive, protective film on the surface to the lithium metal which prevents freshly deposited lithium from undergoing parasitic side reactions with the electrolyte solution. It is further proposed that CO₂ could also be altering the morphology of the plated lithium, making it less dendritic in nature. In any event, however, the beneficial effects of CO₂ appear unique to ester solutions, including methyl formate, and are not present in other typical lithium cell electroyte solutions investigated. This invention, therefore, describes an improved ester-based solution that offers greatly enhanced lithium cycling efficiencies for use in long cycle life rechargeable cells. These improvements are achieved using CO₂ as an electroyte additive and to pretreat the lithium anodes. In addition to the benefits the present invention brings to secondary lithium cells, it should be noted that the invention is also applicable to primary cells. This is because it has been discovered that CO₂ reduces anode polarization in methyl formate solutions and appears to reduce passivation effects.

Claims: A non-aqueous active metal cell comprising an alkali metal anode, an organic ester solvent and an electrolyte salt of the metal of the anode soluble in said solvent, characterized in that the electrolyte solution is saturated with carbon dioxide. The active metal cell of claim 1, characterized in that the alkali metal is lithium. The active metal cell of claim 1 or claim 2, characterized in that the organic ester is methyl formate. The active metal cell of claim 2 or claim 3, characterized in that the electrolyte salt is LiASF₆. The active metal cell of claim 2, 3 or 4, characterized in that the solution also contains LiBF₄. The active metal cell of one of the preceding claims, characterized in that said anode is pretreated with CO₂. The active metal cell of one of the claims 4 to 6, characterized in that said electrolyte salt consists of a combination of LiASF₆ and LiBF₄. The active metal cell of claim 7, characterized in that the electrolyte salt is proportioned according to the relationship XM LiAsF₆ + X 5 M LiBF₄ where: X has a value between 0.25 and 4.0 and M is molarity. The active metal cell of one of the preceding claims, characterized in that said electrolyte solution is a solution of methyl formate containing lithium salt and CO₂.

Inventors: EBNER, WALTER B., LIN, HSIU-PING W.
Applicants: ALLIANT TECHSYSTEMS INC.
Application year: 1989
Main classification: H01M6
IPC codes: H01M4, H01M10