Alkaline galvanic cells

A battery having a zinc or zinc alloy anode, a metal oxide or hydroxide cathode and an alkaline electrolyte comprising a solution of a salt formed by the reaction of boric acid, phosphoric acid or arsenic acid with an alkali or earth alkali hydroxide present in a sufficient amount to produce a stoichiometric excess of hydroxide to acid in the range of 2.5 to 11.0 equivalents per liter, and of a soluble alkali or earth or earth alkali fluoride in an amount corresponding to a concentration range of 0.01 to 1.0 equivalents per liter of total solution.

TECHNICAL FIELD 
This invention relates to alkaline galvanic cells of the types having zinc 
anodes and electrolytes which have a solution of a salt formed by the 
reaction of boric, phosphoric or arsenic acid with an alkali or earth 
alkali hydroxide. 
BACKGROUND OF THE INVENTION 
In U.S. Pat. No. 4,224,391, it was described that the employment of 
electrolytes which contain salts of strong alkali or earth alkali 
hydroxides with weak acids, with a slight excess of hydroxide and a pH 
value of between 9 and 14, reduced the solubility of zinc anode batteries. 
Electrolytes prepared from a mixture of alkali or earth alkali metal 
hydroxide solutions in water and boric acid, phosphoric acid or arsenic 
acid, with excess hydroxide ranging from 0.02 to 3.0 equivalents per liter 
of solution, produces particularly favorable anode performance. Such 
electrolytes substantially eliminate the dual problems of shape-charge and 
dendritic deposition leading to a substantial improvement in the cycle 
life of rechargeable alkaline batteries containing zinc anodes. In that 
patent the combination of the alkali or earth alkali hydroxides and 
selected acids or their equivalent salts, such as borates, metaborates, 
various phosphate or arsenate salts, was taught to result in a net 
stoichiometric excess of the hydroxide in the range of 0.02 to 3.0 
equivalents per liter. 
Considerable improvements in the charge-discharge cycle life of alkaline 
rechargeable batteries, such as nickel oxide-zinc batteries, have been 
reported based on the previous invention. See, for example, M. Eisenberg, 
"A new Stabilized Nickel-Zinc Battery System for Electric Vehicle 
Applications", Paper #830287, Soc. of Automotive Eng., Mar. 4, 1983 and M. 
Eisenberg and J. R. Moden, "New Stabilized Chemistry Nickel-Zinc Cells", 
31st Power Sources Symp., p. 265 (Electrochem. Soc. 1984). With the new 
electrolyte system based on U.S. Pat. No. 4,224,391, nickel oxide-zinc 
cells could be cycled at 80% depth-of-discharge (D.O.D.) up to 600 or more 
cycles compared with a typical 50-200 cycle life for conventional cells 
containing 34-38% by weight potassium hydroxide solutions. 
Unfortunately, as cycling proceeds of rechargeable alkaline batteries of 
the type disclosed in U.S. Pat. No. 4,224,391, gradually the capacity 
yields, expressed in ampere-hours, diminish. Typical capacity losses 
within 500 cycles may amount to 30-50% of the original fresh cell 
capacity. 
Investigations have also been made combining potassium hydroxide with 
potassium fluoride. These are reported in Paper No. 15 of the 10th 
International Power Sources Symposium in Brighton, England, 1976 by N. 
Cenek et al and in U.S. Pat. No. 4,247,610 (1981). However, cycle life and 
capacity results with these have remained limited. 
In the U.S. Pat. No. 4,273,841 (Carlson) a ternary electrolyte composed of 
potassium hydroxide (KOH), potassium phosphate (K.sub.3 PO.sub.4), and 
potassium fluoride (KF) is proposed in concentration ranges of 5-10%, 
10-20% and 5-15%, respectively. Assuming average densities of 1.28-1.37 
g/cc, these concentrations can be translated in terms of molarities as 
follows: 
KOH: 1.14-2.44 ML; K.sub.3 PO.sub.4 : 0.603-1.291 M/L and KF: 1.10-3.54 
M/L. 
These electrolytes however have been found to provide quite limited 
capacity of the nickel oxide electrodes, both in the beginning and later 
on as charge-discharge cycling proceeds. 
SUMMARY OF THE INVENTION 
It has now been discovered that contrary to the teachings of the prior art 
when one employs a phosphate salt such as K.sub.3 PO.sub.4 or borate 
K.sub.3 BO.sub.3 or K.sub.2 NaBO.sub.3, in the molarity range of 0.6 to 
1.3 M/L, that a much fuller realization of the capacity of the nickel-zinc 
cells requires that the concentrations of alkali hydroxide such as KOH 
must be substantially higher than that previously taught and at the same 
time the alkali fluorides must be substantially lower. While it is not 
understood why both of these changes in concentration ranges are 
necessary, it is believed that one beneficially affects the positive 
electrode and the other beneficially affects the negative electrode. 
There are two preferred ways to prepare the new electrolytes. One is to 
create them in situ as originally described in U.S. Pat. No. 4,224,391 by 
reacting strong alkali or earth hydroxides with weak acids such as boric, 
phosphoric or arsenic acid and providing an excess of that hydroxide 
within specified limits or by mixing the corresponding alkali neutral 
salts with the three alkali hydroxides such as KOH, in the same amounts as 
the specified excess molarity ranges. For example, to prepare an 
electrolyte which is 1.0 M/L K.sub.3 PO.sub.4 3 M/L KOH, one can directly 
employ the two compounds in the above concentrations or combine 1 M/L of 
the acid H.sub.3 PO.sub.4 with 6 M/L KOH (3.times.1 M/L+3 M/L). 
A ternary electrolyte system with an alkali hydroxide content of between 
2.5 M/L and 11 M/L in combination with phosphate and borate salts in the 
range of 1.3-2.5 M/L, and in further combination with alkali or earth 
alkali metal fluorides in the limited concentration range of 0.01 to 1.00 
M/L, produces new electrolytes with unexpectedly and substantially 
increased capacity yields of nickel-zinc cells. 
The addition of limited quantities not exceeding the range of 0.01-100 M/L 
of fluorides of alkali or earth alkali metals which are soluble in 
electrolytes that contain salts or strong alkali or earth alkali 
hydroxides with weak acids, such as boric, phosphoric or arsenic acid 
hydroxides, with an excess of the hydroxide, of at least 2.5 M/L, 
substantially enhances the capacity retention of alkaline zinc-anode 
containing cells, as cycling proceeds. 
In a preferred form of the invention, an electrolyte is provided for a 
battery having zinc or a zinc alloy as an active anodic material and a 
metal oxide or hydroxide as an active cathodic material. The electrolyte 
is alkaline and is formed of an alkali or earth alkali metal hydroxide 
mixed with boric acid, phosphoric acid or arsenic acid to produce an 
excess of hydroxide in the range of 2.50-11.0 equivalents per liter of 
solution. The solution also includes dissolved alkali or earth alkali 
fluorides in a concentration of between 0.01 to 1.0 equivalents per liter 
of the total electrolyte solution. The electrolytes can also be prepared 
by using directly the alkali metal borate, phosphate and arsenic salts as 
long as the hydroxide concentration is in the range of 2.5-11.0 
equivalents per liter (or M/L).

DETAILED DESCRIPTION 
Experiment I 
Two electrolytes designated as K-1 and K-2 were prepared in accordance with 
this invention as shown in Table 1. The KOH concentration was 2.56 M/L, in 
both. The potassium phosphate, K.sub.3 PO.sub.4, was 1.66 and 1.47 M/L, 
respectively and the amounts of potassium fluoride, KF, was limited to 
0.34 and 0.68 M/L, respectively. In addition, an electrolyte was prepared 
as specifically described in claim 5 of U.S. Pat. No. 4,273,841 of 
Carlson. The weight percentages given there have been recalculated into 
molarities as shown in Table 1. It should be noticed that the KOH molarity 
of 177 is well below the range specified in the present invention, which 
is 2.5-11 M/L, and that the KF concentration of 2.74 M/L is also well 
above the upper limit of the KF range of the present invention, which is 
0.01-1.00 of M/L. 
Four ampere-hour nominal capacity nickel-zinc cells employing four double 
nickel-oxide cathodes were constructed each 1.7.times.1.75 inches in size 
and 0.035 inches thick. The cells were assembled with zinc anodes of the 
same size and a separator system of non-woven nylon and microporous 
polyethylene film. Groups of three cells each were filled correspondingly 
with electrolytes K-1, K-2 (prepared according to this invention) and 
electrolyte NC-101 prepared according to U.S. Pat. No. 4,273,841. All 
cells were vacuum-filled and allowed to stand for three days to assure 
good wetting of the plates. After initial charging cells were discharged 
at 1 amp to a 1 volt cut-off point. The experiment covered many cycles, 
the first eight of which are summarized in Table 1. 
TABLE 1 
______________________________________ 
COMISON OF CAITY YIELDS OF NICKEL- 
OXIDE-ZINC-CELLS FILLED WITH 3 ELECTROLYTES 
Compositions based on average density of 1.327 g/cc 
Electrolyte Electrolyte Electrolyte 
#K-1 (accord- #K-2 (accord- 
#N C-101 
ing to this ing to this (according to 
invention) invention) Pat. 4,273,841) 
______________________________________ 
KOH 2.56 10.32% 2.56 10.82% 1.77 7.5% 
K.sub.3 PO.sub.4 
1.66 22.03% 1.47 19.51% 1.00 16% 
KF 0.34 1.49% 0.68 2.98% 2.74 12% 
Amp Capacities delivered in discharge to a 
1.0 v/cell cut-off 
Cycle 
#1 4.8-5.35 AH 4.3-525 AH 
1.75-2.05 AH 
#2 4.6-4.9 AH 4.9-5.1 AH 2.00-2.9 AH 
#3 4.65-5.2 AH 4.65-5.2 AH 
2.1-2.4 AH 
#4 4.2-5.3 AH 4.2-4.7 AH 2.5-3.2 AH 
#8 4.5-4.8 AH 4.5-4.8 AH 1.9-2.2 AH 
______________________________________ 
As can be seen in the first cycle, the group of K-1 filled-cells yielded 
4.8-5.35 amp hours (AH). The K-2 group yielded 4.3-5.25 AH. However, the 
NC-101 group yielded 1.75-2.05 AH. In Cycle 2, this last group delivered a 
slightly better capacity of 2 to 2.9 AH, but still far below the 4.6-5.1 
AH values for groups K1 and K2. Even after eight cycles, this situation 
did not change and in subsequent cycling the capacity yields of the cells 
in the last group remained in the range of 1.9 to 2.2 AH compared to 4.5 
to 4.8 for electrolytes K1 and K2 prepared in accordance with the present 
invention. 
Experiment II 
An electrolyte was prepared from an 8.08 moles per liter (8.08 chemical 
equivalents per liter) solution of potassium hydroxide to which boric acid 
was added in the amount of 1.50 moles per liter (4.50 chemical equivalents 
per liter). This provided formation of a solution of 1.50 moles per liter 
of potassium borate and a 3.58 moles per liter of excess potassium 
hydroxide. This solution was designated as Solution #1. To Solution #1 was 
added potassium fluoride (KF) in an amount which resulted in a 0.8 moles 
per liter concentration. This solution was designated as Solution #2. 
Finally, a conventional potassium hydroxide solution of 34% by weight of 
KOH which corresponds to 8.08 moles per liter was prepared and designated 
as Solution #3. 
TABLE 2 
______________________________________ 
Solution 
Content Cycle No. Average Capacity, AH 
______________________________________ 
1 KOH 8 4.1 
Borate 30 3.7 
92 3.0 (75%) 
2 KOH 8 3.9 
Borate 30 3.8 
KF 92 3.7 (93%) 
3 KOH 8 4.3 
Only 30 3.5 
92 (shorted) 
______________________________________ 
It is clear from these results that after 92 cycles the Solution #1 gave an 
80% capacity retention but solution #2 with both borate and KF gave a 93% 
retention. The KOH Solution #3 resulted in cells shorting by zinc 
dendrites before cycle 92 was reached. Hence, the combination of the 
potassium hydroxide and borate and potassium fluoride (as represented by 
Solution #2) yielded the best retention of cell capacity after the 
extended cycling even so the capacity may have been somewhat lower in the 
initial cycle, for instance, in Cycle #8. 
Experiment III 
In this experiment larger nickel oxide zinc cells of a nominal capacity of 
16-20 ampere-hours (AH) were employed. Again, three solutions were 
employed. Solution #4 contained 1.9 moles per liter (m/L) of potassium 
borate and 2.6 m/L potassium hydroxide. Solution #5 contained 0.8 m/L 
potassium fluoride (KF) and 3.3 m/L potassium hydroxide (KOH). Solution #6 
contained 0.8 m/L potassium fluoride 2.8 m/L KOH and 0.9 m/L borate 
(K.sub.3 BO.sub.3) In addition, all three of these solutions contain 0.2 
m/L lithium hydroxide (LiOH). Three groups of three cells each were cycled 
at 80% depth of discharge using a 9 hour charge and 3 hour discharge. 
Table 3 gives the average cell capacities after a number of cycles. 
TABLE 3 
______________________________________ 
AVERAGE NICKEL ZINC CELL CAITIES (AH) 
Cycle No. Solution 4 Solution 5 Solution 6 
______________________________________ 
4 20 AH 8.7 AH 10.4 AH 
79 19 AH 13 AH 16 AH 
188 19 AH 18 AH 22 AH 
283 17 AH 20 AH 23 AH 
______________________________________ 
From this it is clear that the combination of the three constituents, as 
represented by Solution #6, gave the best overall results except in the 
initial cycle. Particularly impressive were the results after cycle #79. 
Experiment IV 
Two groups of three nickel zinc cells, each as described in Experiment III, 
were filled with Solution #1 from Experiment II, KOH and borate and a 
modified Solution #7 containing in addition 0.3 M/L phosphate, K.sub.3 
PO.sub.4 and 0.3 M/L sodium fluoride (NaF). The two groups of cells were 
cycled to a 100% DOD. At cycle #137 the group with Solution #1 showed an 
average 74% capacity retention. However, the group with Solution #7 
averaged an 85% retention of cell capacity. 
Experiment IV 
Nine small silver oxide-zinc cells of a nominal capacity of 500 
milliampere-hours (ma) were divided into three groups of 3 cells each and 
after the second discharge, subjected to automatic cycling to an 80% depth 
DOD. The first group of three cells was filled with Solution #8 which 
contained 11.6 M/L KOH. A solution #9 contained 9.3 M/L KOH and 0.5 M/L 
K.sub.3 BO.sub.3. Finally, a solution #10 in the third group contained 9.3 
M/L KOH, 0.5 K.sub.3 BO.sub.3 and 0.12 M/L potassium fluoride. The results 
of the tests are given in Table 4. 
TABLE 4 
______________________________________ 
AVERAGE CAITIES (ma) OF SILVER-ZINC 
CELLS FOR THREE ELECTROLYTES (80% DOD) 
Solution 8 
Solution 9 Solution 10 
______________________________________ 
Compositions M/L 
KOH 11.6 KOH 9.3 KOH 9.3 
K.sub.3 BO.sub.3 0.5 
K.sub.3 BO.sub.3 0.5 
KF 0.12 
Cycle # 
2 650 mAH 600 mAH 580 mAH 
13 504 565 585 
63 120 & 2 510 556 
shorted 
123 -- 392 (65%) 430 (74%) 
______________________________________ 
As can be seen Solution #8 with potassium hydroxide only (KOH) gave in the 
initial cycle #2 the highest capacity. However, by cycle #13 it already 
dropped from 650 to 504 ma and by cycle #63 one cell delivered only 120 
mAH and two cells were already shorted. Since the 45% KOH solution is a 
currently accepted standard for silver oxide zinc cells, the effects of 
the borate and fluoride additives in Solutions #9 and #10 can be 
appreciated. As shown in Table 4, these two groups average capacities 
still in excess of 500 ma in cycle #63 and even in cycle #123 delivered 
respectable fractions of original capacity. In this generally low cycle 
life rechargeable battery system, it is also interesting to note that 
Solution #10, containing both the fluoride and the borate, provided in 
cycle #123, at 75% capacity retention compared to 65% for the Solution #9 
with only the borate. It should also be noted that with KOH alone silver 
oxide-zinc cells rarely exceed 40-60 cycles at 80% DOD. 
Experiment VI 
Two groups of similar cells as described in experiment 4 were filled with 
electrolytes 8 and 11, the compositions of which are given in Table 6. The 
cells were subject to full discharge, i.e. 100% DOD. 
TABLE 6 
______________________________________ 
AVERAGE CAITIES (ma) OF SILVER-ZINC 
CELLS WITH TWO ELECTROLYTES-CYCLED 100% DOD 
Solution 8 
Solution 11 
______________________________________ 
Composition m/L KOH 11.6 KOH 9.5 
K.sub.3 BO.sub.3 0.5 
Cycle # 
3 636 ma 610 ma 
17 505 550 
57 0.170 460 
(shorted) 
93 -- 363 
______________________________________ 
The group with the Solution #8 (standard) did not reach cycle #57, failing 
by dendrite shorting. The group with Solution #11 with borate survived at 
least to cycle #93, at which point it still averaged a capacity of 363 ma. 
It should be understood that the just described embodiments merely 
illustrate principles of the invention in its preferred forms. Many 
modifications, additions, and deletions may, of course, be made thereto 
without departure from the spirit and scope of the invention as set forth 
in the following claims.