Ternary electrolyte for secondary electrochemical cells

An aqueous alkaline solution comprised of 5 weight % to 10 weight % KOH, 5 weight % to 15 weight % KF and 10 weight % to 20 weight % K.sub.3 PO.sub.4 is used as an electrolyte in a secondary electrochemical cell wherein the active material of one of the electrodes is a zinc-active material.

This invention relates to an aqueous alkaline ternary KOH/KF/K.sub.3 
PO.sub.4 solution for use as an electrolyte in a secondary electrochemical 
cell wherein the active material of one of the electrodes is a zinc-active 
material. 
Secondary electrochemical cells with alkaline electrolyte and zinc negative 
electrodes have limited cycle life because of the zinc electrode, i.e. 
because of redistribution of the zinc active material. Specifically, 
because the zinc discharge products are highly soluble in the electrolyte, 
secondary zinc electrodes suffer from slumping, shape change, 
densification and dendrite growth. On recharge, the zinc active material 
does not replate in its original form and location, and under some 
charging conditions, plates in the form of dendrites which can short to 
the positive plate. 
One approach to alleviating these problems is to modifiy the electrolyte 
(usually 4 to 10 M KOH) to reduce the solubility of the zinc discharge 
products. A modified electrolyte must allow reasonable rates of charge and 
discharge of the zinc and the positive electrodes over a useful 
temperature range and possess sufficient ionic conductivity to allow a 
finished cell to have a low internal resistance. 
Ser. No. 067,696, now abandoned and Ser. No. 067,695, both filed Aug. 20, 
1979 in the name of R. F. Thornton and assigned to the assignee hereof, 
disclose a binary electrolyte. Ser. No. 067,696 discloses an electrolyte 
of 5 to 18% phosphoric acid (H.sub.3 PO.sub.4) and 15 to 31% potassium 
hydroxide (KOH), and Ser. No. 067,695 discloses 18 to 30% potassium 
fluoride (KF) and 15 to 2% potassium hydroxide (KOH), respectively. 
Although each of the disclosed binary electrolytes shows a decrease in 
zinc solubility, dendrite formation and electrode shape change, there was 
also a decrease in rate capability and cell discharge capacity. 
The present aqueous KF/K.sub.3 PO.sub.4 /KOH electrolyte retains the 
ability of the aforementioned binary electrolytes to inhibit slumping, 
shape change, densification and dendrite growth by also having a low 
solubility for the zinc discharge products. However, the present ternary 
electrolyte shows a significant improvement in performance over the 
aforementioned binary electrolytes. Specifically, the present ternary 
electrolyte exhibits the charge characteristics of the aforementioned 
KOH/KF electrolyte and the discharge characteristics of the aforementioned 
KOH/K.sub.3 PO.sub.4 electrolyte. 
Briefly stated, the present invention is directed to a secondary 
electrochemical cell wherein the active material of one of the electrodes 
is a zinc-active material and wherein the elctrolyte is comprised of an 
aqueous solution of 5 to 10 weight % potassium hydroxide, 5 to 15 weight % 
potassium fluoride and 10 to 20 weight % potassium phosphate with the 
remaining weight % being water. 
In the present electrolyte solution, KOH in an amount higher than 10 weight 
% is not useful because of its deteriorating effect on the negative zinc 
electrode whereas amounts of KOH lower than 5 weight % are not practical 
due to the resulting decrease in rate capability and capacity. Amounts of 
KF lower than 5 weight % cause a deterioration of charge characteristics 
of the electrolyte whereas reduction of K.sub.3 PO.sub.4 below 10 weight % 
diminishes rate capability. On the other hand, the dissolved salt content 
of the present electrolyte should not exceed about 45 weight % since with 
a salt content higher than about 45 weight %, conductivity of the 
electrolyte is decreased and low temperature performance suffers. 
The particular formulation of the present electrolyte depends largely on 
the specific battery properties desired. Preferably, the present 
electrolyte is an aqueous solution of 7.5 weight % potassium hydroxide, 
12.0 weight % potassium fluoride and 16.0 weight % potassium phosphate. 
As used herein active material is electrode material that undergoes 
electrochemical oxidation or reduction. 
Representative of the zinc active material of the negative electrode in the 
present secondary electrochemical cell is elemental zinc, or a compound of 
zinc such as zinc oxide, or mixtures thereof. 
The present electrolyte is particularly useful in a secondary 
electrochemical cell wherein the active material of the positive electrode 
is preferably NiOOH/Ni(OH).sub.2.

EXAMPLE 
A number of aqueous electrolyte solutions were prepared, and their 
compositions, expressed in weight %, are given in Table I. 
TABLE I 
______________________________________ 
Composition (wt %) 
Run KOH KF K.sub.3 PO.sub. 4 
______________________________________ 
1 31 X X 
2 4.8 26.8 X 
3 7.5 24.3 X 
4 4.3 X 32.7 
5 7.5 12.0 16.0 
______________________________________ 
The performance characteristics of the electrolytes listed in Table I were 
compared by cycling the electrolytes in identically constructed positive 
limited Ni/Zn cells with a theoretical positive capacity equal to 0.79 AH 
and a negative capacity of 1.52 AH. Specifically, the zinc electrode had 
an initial formulation comprised of a mixture of zinc oxide and a minor 
amount of elemental zinc powder with a polymer binder in an amount of 2 
weight % or less of the mixture. The active material of the positive 
electrode was NiOOH/Ni(OH).sub.2. 
The discharge characteristics and total capacity were determined at room 
temperature by subjecting the cells to a step discharge (successive 
discharges of 1 ampere, 200 mA, 40 mA, and 10 mA; each with a voltage 
cutoff of 1.00 volts) following the initial formation charge. The results 
of these discharges are summarized in FIG. 1. 
Run 5 utilizing the present ternary electrolyte illustrates the present 
invention. Specifically, FIG. 1 shows that the ampere capacity of Run 5 is 
approximately twice that of the KOH/KF electrolytes of Runs 2 and 3, and 
equivalent to the KOH/K.sub.3 PO.sub.4 electrolyte of Run 4. Also, the 
total capacity of the present ternary electrolyte of Run 5 equals that of 
the binary electrolyte of Run 3 and that of KOH of Run 1 (small 
differences exist because of variations in the formation charge) FIG. 1 
also shows that the KF/KOH electrolyte of Run 2 and the KOH/K.sub.3 
PO.sub.4 electrolyte of Run 4 have significantly less total capacity than 
that of the present electrolyte. 
The polarization (voltage vs. capacity) curves of the zinc electrode in the 
different electrolytes during the 1 ampere discharge are compared in FIG. 
2. Specifically, FIG. 2 shows that the present ternary electrolyte has 
similar discharge characteristics as the KOH/K.sub.3 PO.sub.4 electrolyte 
of Run 4. At the same time the ternary electrolyte polarizes less than the 
binary KF/KOH electrolytes of Runs 2 and 3. It was determined that the 
positive (nickel) electrode behaved in a similar fashion in all the 
electrolytes. 
The polarization curves for the zinc electrode for the charge following the 
step discharge are given in FIG. 3. The ternary electrolyte, i.e. Run 5, 
has intermediate charge acceptance characteristics between the KOH/K.sub.3 
PO.sub.4 of Run 4 and the KOH/KF electrolytes of Runs 2 and 3. The current 
during the initial period of the constant current charge was 10 mA. This 
was then raised to 50 mA (after .about.0.02-0.05 AH). The cells were 
charged at 50 mA until the total cell voltage exceeded 1.90 volts, at 
which time the charge was stopped. 
In both charge and discharge, the behavior of the zinc electrode varied as 
the electrolyte was changed. Specifically FIGS. 2 and 3 show that the 
ternary electrolyte combines the discharge behavior of the KOH/K.sub.3 
PO.sub.4 binary electrolyte and the charge characteristics of the KOH/KF 
binary solutions. 
The positive electrode behaved similarly in all the electrolytes.