CA1131306A

CA1131306A - Tetrafluoroborate additive for high drain rate lithium cells

Info

Publication number: CA1131306A
Application number: CA335,955A
Authority: CA
Inventors: Nehemiah Margalit; Philip E. Krouse
Original assignee: ESB International Corp
Current assignee: ESB International Corp
Priority date: 1978-09-25
Filing date: 1979-09-19
Publication date: 1982-09-07
Also published as: DK398079A; JPS5546298A; DE2938461A1

Abstract

ADDITIVE FOR HIGH DRAIN RATE LITHIUM CELLS

Abstract An improved high drain rate lithium cell is provided. The improvement is the use of an additive to stabilize the cell. The additive is a minor portion of a tetrafluoroborate salt of sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, barium, aluminum or mixtures thereof.

Description

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ADDITrVE FOR HIGH DRAIN RATE LITHIl~l CELLS
Backgrou~d of the Invention 1. Field of the Invention This invention relates to an additive for non-aqueous elec-trolyte of a high rate lithium electrochemical cell. More particularly, r it relates to the use of certain alkali metal tetrafluoroborates as additives for sta~ilizing ehe cell.
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2. Prior Art It has been known for some time that high rate drain cells having anodes of lithium and electrolyte of solvent and solute are not stable, particularly at high temperatures.
The instability probIem is three fold. First, there is a short discharge life after storage due to apparent chemical changes in the electrolyte; secondly, there is a pressure build-up because of detrimental side reactions in the electrolyte; and thirdly, there are cell degradation reactions among cell components. Yet these same unstable electrolyte systems for lithi~m batteries produce extremely high energy due to high conductivity of the solute in the solvent.
In lithium cells, cell degradation reactions can be manifested in various ways: crust formation on the anode, eg., with SOC12 solutions;
gas formation and cell rupture, eg., with lithium hexafluoroarsenate in methylformate, and polymerization of solvent, eg., in THF solutions.
- In all three type degradations, the reaction leads to decreased perform-ance, if not complete failure, of the cell. When using non-aqueou~
solvents in the electrolyte, both gassing and formation of viscous, if not solid, reaction products near the anode are of maJor concern; stable or stablized electrolyte has to be used in lithium cells.

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There are many applications, eg., watch, camera, etc., for which batteries must be constructed in a shape requiring high conductivity to permit efficient and economical utilization of the chemicals. Design limitations require utilization of highly conductive electrolytes. Ho~ever, conditions of use require electrolytes capable of long term storage.
In United States Patent 3,887,397, issued June 3, 1975, to Robert J~ Horning, it is disclosed that in lithium cells having a highly conductive methylformate/lithium hexafluoroarsenate electrolyte, the addition of small amounts of lithium tetrafluoroborate will significantly improve the stability of the cell. Thus by using lithium tetrafluoroborate as a stabilizer, Horning succeeded in practically eliminating the gassing reaction between lithium and the specific electrolyte.
However, no mention is made in the patent of any other tetra-fluoroborate which could be utilized or of any ability to control non-gassing reactions causing deposition of reddish-brown viscous products on and near the lithium. Such viscous deposits will hinder the performance of the anode after prolonged storage andior at higher rates by slowing ionic mobility near the anode.
It is the objective of this invention to provide a method of stabilizing non-aqueous lithium batteries in terms of reducing chemical interactions between the electrolyte and cell components.
A more particular object of this invention is to provide stabilizing additives for high rate lithium cells so that gassing and other reaction product formations are not observed.
Summary of the Invention It has been discovered that the stability of high rate batteries which contain a lithium anode, an electrolyte having a solvent and solute and a cathode will be significantly improved by the addition to the electrolyte of a minor portion of a tetrafluoroborate salt of sodium, potassium, or mixtures thereof dissolved in said electrolyte, wherein the amount of tetrafluoroborate salt ranges from 0.05 to 0.1 molar.

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Detailed ~escription of the Invention The cathode can be any cathodic material which i8 inert to the electrolyte, among these are V205, Ag2CrO4, (CF)n, ~nO2, CuS and others which are known to those skilled in the art. As stated above, this invention relates basically to stabilizing of the electrolyte and anode, hence, does not require any particular cathode for use.
The solvents of the electrolyte used in the current invention are those which are known for use in high rate cells. Examples include methylformate, gamma butyrolactone, 1,2 - dimethoxyethane and 1,3 -dioxolane, and mixtures thereof.
The solutes which are dissolved in the solvents to form the electrolyte are also known to those skilled in the art. Examples include the hexafluoroarsenates, the trifluoromethylsulfonates and the perchlorates. It is preferred that the salt of these cations be the lithium salt.
The additive is a tetrafluoroborate salt of sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, barium, aluminum or mixtures thereof. The sodium and potassium salts are preferred.
The amount of the tetrafluoroborate salt which should be employed varies depending upon the stability desired. Normally, the amount of tetrafluoroborate salt will range from approximately 0.05 molar to as much as 0.8 molar in the electrolyte. A more preferred range is about from 0.05 to 0.1 molar percent.
The current inventors have found that the use of these tetra-fluoroborate salts not only stops the gas reaction but also decreases and practically eliminates~the non-gassing side reactions`. For,example, in the case of lithium hexafluoroarsenate in methylformate with lithium tetrafluoroborate or no additives, these side reactions produce a reddish-brown viscous product that clings to the lithium. This product will effect the high rate performance of cells by both slowing ionic bility near the anode. The current inventors have found that even in

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highly reactive llthlu~ hexsfluoroarsenate in methylformate electrolyte, the elimination of the ~de reaction is increased if one uses the tetra-fluoroborate salt of sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, barium, aluminum or mixtures thereof.

EX~MPLE 1 Strips of lithium were placed in four heavy glass tubes each having a different electrolyte solution. The tubes were part of an apparatus used to measure the gassing and observe corrosion on the lithium. The apparatus consisted of a heavy glass tube (Fischer and Porter Co., Aerosol Reaction Vessel, No. 110-205) connected to an Bourdon gauge (-30 to 150 inches mercury, U.S. Gauge No. 5060). The seal between the gas vessel and tube leading to the gauge was made with a stainless steel adapter plug fitted with a silicone rubber ~0~! ring.
The sealed apparatus was placed into an oven at 165F and monitored daily for gassing and for impurity formation on the lithium.
The experiments were continued until there was a significant increase in the pressure within the test tubes of 1 to 1-1/2 atmospheres of pressure over the pressure of equilibrium at 165F or until they extended over a period of time in which a significant difference could be seen in the additives.
In all the tests the solvent was methylformate and the solute was 2.0 molar LiAsF6. The electrolyte differed only by additive. The results were as foll~ws:

Hours Until Visual Significant Inspection Additive Pressure Build IT~ Of Lithium None 787 Substantial Amount of Viscous Material O.IM LiBF4 984* Large Amount of Viscous Material 0.1~ NaBF 984* Substantially Free of

4 Viscous Material O.1~ KBF4 984* Substantially Free of Viscous ~aterial * ~he test was deliberately terminated at this point.

Claims

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improvement in an electrochemical cell having a lithium anode, a non-aqueous electrolyte comprised of a solvent and solute and a cathode, the improvement comprises having as an additive a minor portion of a tetra-fluoroborate salt of sodium, potassium, or mixtures thereof dissolved in said electrolyte, wherein the amount of tetrafluoroborate salt ranges from 0.05 to 0.1 molar.

2. The cell of claim 1 wherein the electrolyte is methylformate, gamma-butyrolactone, 1,2-dimethyoxyethane, 1,3-dioxolane or mixtures thereof.

3. The cell of claim 1 wherein the solvent is methylformate.

4. The cell of claim 1 wherein the solute is lithium hexafluoro-arsenate, lithium trifluoromethylsulfonate or lithium perchlorate.

5. In a current producing cell having a lithium anode, an electrolyte of methylformate having lithium hexafluoroarsenate dissolved therein and a cathode, the improvement comprising having a minor portion of sodium tetra-fluoroborate dissolved in said electrolyte, wherein the amount of tetrafluoro-borate salt ranges from 0.05 to 0.1 molar.

6. In a current producing cell having a lithium anode, an electrolyte of methylformate having lithium hexafluoroarsenate dissolved therein and a cathode, the improvement comprising having a minor portion of potassium tetrafluoroborate dissolved in said electrolyte, wherein the amount of tetrafluoroborate salt ranges from 0.05 to 0.1 molar.