IE43292B1 - An electrochemical cell - Google Patents

Abstract

1489730 Primary cells SAFT-SOC DES ACCUMULATEURS FIXES ET DE TRACTION 14 Sept 1976 [15 Sept 1975] 38091/76 Heading H1B The active material of a positive electrode for a cell having, e.g. a Li negative electrode and an electrolyte of lithium perchlorate in propylene carbonate, comprises a monovalent silver ferro or ferric cyanide.

Description

The present invention relates to an electrochemical cell, particularly a primary electrochemical cell having high energy density, and is particularly concerned with the active material of the positive electrode of such a cell which preferably has a lithium anode and an electrolyte constituted by a solution in which the solvent is an aprotic compound.

Positive active materials for electrochemical cells, which materials are constituted by oxygenated metal salts whose anion is complex and is highly co-ordinated by the oxygen as ligand, are already known. The central atom of the anion can be a metalloid (phosphorus or arsenic for example) or a metal (chromium, molybdenum or vanadium for example). Certain cells having positive active material constituted by silver chromate may produce discharges at a low rate and a stability such that they are - 2 43292 suitable for use in watches or heart stimulators (pacemakers). Although such positive active material has proved satisfactory, substitute positive active materials have been sought which could be used in the case of a shortage of silver chromate, the preparation of which is fairly difficult.

According to the present invention, there is provided an electrochemical cell having a positive electrode whose active material comprises a metal salt whose cation is monovalent silver and whose complex anion is a ferrocyanide or a ferricyanide.

Preferably such a cell has a negative electrode comprising lithium and an electrolyte constituted by a solution having as its solvent at least one aprotic compound.

The present invention will be better understood with reference to the examples described herebelow with reference to the accompanying drawing, in which the single figure shows the discharge curves of two primary electrochemical cells according to the present invention.

EXAMPLE A A primary electrochemical cell was formed having lithium for the negative electrodes and a molar solution of lithium perchlorate in propylene carbonate for the electrolyte. The positive active material was constituted by silver ferricyanide, (FeiCNJJAg^ and was mixed with graphite to obtain acceptable conductivity and with a fluorinated polymer, for example, polytetrafluoroethylene, which acted as a binding agent. The proportions of this mixture were: Silver ferricyanide 100 Graphite 20 Fluorinated polymer 20 - 3 - A pellet of 2 g of this mixture was compressed on a silver collector made of expanded metal sheet, at a pressure of about 0.6 tonne per square centimetre to form a positive electrode having circular cross-section of area about 5 sq.cm.

As this electrode has two circular faces and small thickness, the active surface area is about 10 sq.cm. Two disc-shaped negative electrodes of lithium having a diameter of about 3.2 cm and a thickness of at least 0.6mm were disposed one on either side of the positive electrode, separators being inserted between the positive electrode and each of the negative electrodes. Each separator comprises two layers, one made of a rag paper and in contact with the negative electrode and the other made of cotton fibre felt. The distance between the positive electrode and each negative electrode was about 5 mm. The whole was immersed in the electrolyte.

The curve 1 in the figure shows the discharge of this primary cell at a constant current of 0.1 mA per sq.cm., i.e. a total current of 1 mA, the voltage V (volts) (along the ordinate) being shown as a function of the discharge time T (hours) (along the abscissa).

As may be seen, the curve begins at 3.4 volts and decreases rapidly to become stabilized after about 18 hours at a voltage level between 2.7 and 2.9 volts. This part of the curve corresponds to a ferricyanide-ferrocyanide reduction !5 according to the reaction: {Fe(CN)g}3- +e+ {Fe(CN)g}4The discharge of the active material subsequently corresponds to the discharge of the silver up to valency 0 according to the reaction: Q {Fe(CN)g}Ag4+4e+ 4Ag+{Fe(CN)g}4- EXAMPLE B This result was confirmed by the discharge of another primary electrochemical cell in which the silver - 4 43392 ferricyanide was replaced by silver ferrocyanide (Fe(CNg))Ag4, in the same proportions, the other components remaining identical. The curve of the discharge of this primary cell is shown in the figure by the dashed line curve 2.

As may be seen, the discharge is level from the beginning of the curve, because of the prior preparation of the silver ferrocyanide.

The use of silver ferrocyanide therefore seems preferable to the use of ferricyanide if it is required to avoid initial over-voltage due to the reducing of the ferricyanide ion such as takes place in the cell of Example A.

The positive active materials described in these examples are capable of a performance at least equal to that of the known positive active materials constituted by salts in which the liquid of the anion is oxygen.

Claims (5)

1. An electrochemical cell having a positive electrode whose active material comprises a metal salt whose cation is monovalent silver and whose complex 5 anion is a ferrocyanide or a ferricyanide.

2. An electrochemical cell according to claim 1, whose negative electrode comprises lithium and whose electrolyte is constituted by a solution having as its solvent at least one aprotic compound. 10

3. An electrochemical cell according to claim 2, wherein the electrolyte is constituted by a solution of lithium perchlorate in propylene carbonate.

4. Z\n electrochemical cell substantially as hereinbefore described with reference to Example A or Example B. 15

5. A battery of electrochemical cells according to any one of claims 2 to 4.