Abstract The growth of zinc dendrites during charge of a silver-zinc cell is a particularly undesirable feature of the zinc electrode as it eventually leads to shorting of the cell through growth of metallic trails of zinc through the separator. A method is disclosed of preventing the growth of zinc dendrites and more particularly of preventing zinc dendrite penetration of the separator. The method involves controlling the overpotential in the cell, i.e., limiting the overpotential to a value below the critical magnitude at which the zinc deposits as dendrites instead of as a moss. When zinc deposits as a moss, penetration of the separator does not occur. The new method involves so limiting the charge overpotential and using a charge rate that the cell is fully charged by the time the critical overpotential is reached. The overpotential is measured between the working zinc electrode and a built-in zinc reference electrode. When the critical overpotential is reached, charging is terminated. The subject matter of the invention also includes an improved electrochemical apparatus whose efficiency and useful life derive from the prevention of zinc penetration of the separator.
A method as defined in claim 1 wherein the source of reference potential is a further electrode consisting essentially of the same active material as said negative electrode, said further electrode being positioned in contact with said electrolyte but excluded from the charging circuit.
Claim 2
Dependent Claim
13 words
A method as defined in claim 2 wherein said active material is zinc.
Claim 3
Dependent Claim
40 words
A method as defined in claim 3 wherein charging is discontinued upon the potential difference between said negative electrode and said further electrode reaching a value corresponding to an overpotential of substantially 50 to 75 millivolts for said negative electrode.
Claim 4
Dependent Claim
34 words
A method as defined in claim 1 wherein charging is discontinued upon the difference of said potentials reaching a value corresponding to an overpotential of substantially 25 to 100 millivolts for said negative electrode.
Claim 5
Dependent Claim
20 words
A method as defined in claim 1 wherein the active material of said negative electrode is zinc, lithium or tin.
Claim 6
Dependent Claim
147 words
In an electrochemical system including a negative electrode with an active material prone to dendritic growth upon overcharging, a positive electrode, and an electrolyte contacting both said electrodes, the combination therewith of a third electrode in contact with said electrolyte but insulated from the other electrodes for generating a reference potential independent of said positive electrode, said third electrode consisting essentially of the same active material as said negative electrode, means including a cutoff switch for connecting said negative and positive electrodes in a charging circuit across a supply of unipolar current, and voltage-measuring means connected between said negative electrode and said third electrode for determining the potential difference therebetween, said voltage-measuring means being connected to said switch for interrupting said charging circuit upon said potential difference reaching a predetermined magnitude corresponding to an overpotential of said negative electrode beyond which dendrite formation is liable to occur.
Claim 7
Dependent Claim
12 words
The combination defined in claim 7 wherein said active material is zinc.
Claim 8
Dependent Claim
29 words
The combination defined in claim 7 wherein said positive electrode, said negative electrode and said third electrode are closely juxtaposed, further comprising separator means interposed between said juxtaposed electrodes.