Linden D., Reddy T.B. (eds.) Handbook of batteries

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MAGNESIUM WATER-ACTIVATED BATTERIES 17.5

FIGURE 17.2 (c) Recirculation voltage control (Continued).

3. Dunk-type batteries are designed with an absorbent separator between the electrodes and

are activated by pouring the electrolyte into the battery, where it is absorbed by the

separator. Batteries of this type have been designed to produce from 1.5 to 130 V at

currents up to about 10 A. Lengths of discharge vary from about 0.5 to 15 h. Figure 17.3

is a diagrammatic representation of a magnesium/ cuprous chloride battery used in radio-

sonde applications. A pile-type construction is used. A sheet of magnesium is separated

from the cuprous chloride cathode by a porous separator which also serves to retain the

electrolyte. The cathode is a pasted type made by applying a paste of powdered cuprous

chloride and a liquid binder onto a copper grid or screen. The assembly is taped together

to form the battery. The batteries are also made in spiral or jelly-roll design. (See Figure

17.22 for an illustration of this battery.)

FIGURE 17.3 Diagrammatic representation of magnesium / cuprous

chloride dunk-type battery. (Courtesy of Eagle-Picher Industries.)

17.6 CHAPTER SEVENTEEN

FIGURE 17.4 Basic water-activated cell.

17.4 CONSTRUCTION

Water-activated cells consist of an anode, a cathode, a separator, terminations, and some

form of encasement. A battery consists of a multiplicity of cells connected in series or series-

parallel. Such an assembly requires a method to connect the cells in the desired conﬁguration

plus a method to control leakage currents. The voltage of a cell depends primarily on the

electrochemical system involved. To increase voltage, a number of cells must be connected

in series. The capacity of a cell in ampere-hours is primarily dependent on the quantity of

active material in the electrodes. The ability of a cell to produce a given current at a usable

voltage depends on the area of the electrode. To decrease current density so as to increase

load voltage, the electrode area must be increased. Power output depends on the temperature

and salinity of the electrolyte. Power output can be increased by increasing the temperature

or the salinity of the electrolyte.

The basic components of a single cell, a duplex assembly for connecting cells in series,

and a ﬁnished battery are illustrated in Figs. 17.4, 17.5, and 17.1, respectively.

12–16

The

illustrations represent batteries designed for use by immersion in the electrolyte as contrasted

to a dunk-type (radiosonde) battery, which is activated by pouring the electrolyte into the

battery, or a forced-ﬂow electric torpedo battery. The construction principles with slight

variations are similar in all cases.

MAGNESIUM WATER-ACTIVATED BATTERIES 17.7

FIGURE 17.5 Duplex electrode assemblies. (a) Silver. (b) Nonsilver.

17.4.1 Components

A more detailed description of the various cell and battery components and construction

elements follows.

Anode (Negative Plate). The anode is made from sheet magnesium. Magnesium AZ61A

is preferred because it tends to sludge and polarize less. In some cases AZ31B alloy is used;

however, this alloy gives slightly lower voltage, polarizes at high current densities, and

sludges more. In recent years magnesium alloys AP65 and MTA75 have been developed and

17.8 CHAPTER SEVENTEEN

evaluated. These are high-voltage alloys giving load voltages of 0.1–0.3 V higher than

AZ61A. MTA75 is a higher-voltage alloy than AP65. These alloys sludge more; however,

under some forced-ﬂow discharge conditions, the sludging problem may be controlled. These

alloys are not used extensively in the United States; however, they are used in the United

Kingdom and Europe in electric torpedo batteries. Composition ranges of these alloys are

shown in Table 17.2.

TABLE 17.2 Composition Range for Battery Plate Alloys

Element

AZ31

% Min. % Max.

AZ61

% Min. % Max.

AP65

% Min. % Max.

MELMAG 75

% Min. % Max.

2.5

0.6

—

0.15

—

3.5

1.4

—

0.7

0.1

0.04

0.05

0.005

0.006

5.8

0.4

—

0.15

—

0.05

—

7.2

1.5

—

0.25

0.05

0.3

0.05

0.005

0.006

6.0

0.5

4.4

—

0.15

—

0.005

—

6.7

1.5

5.0

—

0.30

0.3

—

0.005

0.010

4.6

—

6.6

—

0.3

—

5.6

0.3

—

7.6

0.25

0.3

—

0.005

0.006

Cathode (Positive Plate). The cathode consists of a depolarizer and a current collector.

These depolarizers are powders and are nonconductive. In order for the depolarizer to func-

tion, a form of carbon is added to impart conductivity; a binder is added for cohesion, and

a metal grid is used as a current collector, a base for the cathode, to facilitate intercel

connections and battery terminations. Possible cathode formulations are shown in Table

17.3

1,3–5,8

TABLE 17.3 Cathode Compositions

Silver

chloride

Cuprous

iodide

5,6

Cuprous

thiocyanate

Lead

chloride

Cuprous

chloride

Depolarizer, %/ w 100 73 75–80 80.7–82.5 95–100

Sulfur, %/ w — 20 10–12 — —

Additive, %/ w — — 0–4 2.3–4.4 —

Carbon, % / w — 7 7–10 9.6–9.8 —

Binder, %/ w — — 0–2 1.5–1.6 0–5

Wax, %/ w — — — 3.8 —

Silver chloride is a special case. Silver chloride can be melted, cast into ingots, and rolled

into sheet stock in thicknesses from about 0.08 mm up. Since this material is malleable and

ductile, it can be used in almost any conﬁguration. Silver chloride is nonconductive and is

made conductive by superﬁcially reducing the surface to silver by immersion in a photo-

graphic developing solution. No base grid need be used with silver chloride.

Nonsilver cathodes are usually prismatic in shape and are ﬂat. Silver chloride cathodes

are used ﬂat and corrugated in many conﬁgurations.

MAGNESIUM WATER-ACTIVATED BATTERIES 17.9

Separators. Separators are nonconductive spacers placed between the electrodes of im-

mersion- and forced-ﬂow-type batteries to form a space for free ingress of electrolyte and

egress of corrosion products. Separators in the form of disks, rods, glass beads, or woven

fabrics may be used.

13–14

Dunk-type batteries utilize a nonwoven, absorbent, nonconductive material for the dual

purpose of separating the electrodes and absorbing the electrolyte.

Intercell Connections. In a series-arranged battery of pile construction, the anode of one

cell is connected to the cathode of the adjacent cell. To accomplish this without producing

a short-circuited cell, an insulating tape or ﬁlm is placed between the electrodes on nonsilver

batteries. For silver batteries silver foil is used alone or in conjunction with an insulating

tape.

For nonsilver cells the connection is made by stapling the electrodes together through the

insulator.

For silver cells, the silver chloride, surface-reduced to silver, is heat-sealed to

silver foil, which has been previously welded to the anode. Where large surface areas are

involved, contact between silver and silver foil can be made by pressure alone.

Terminations. For silver chloride cathodes the lead is soldered directly to silver foil, which

has been heat-sealed to one surface of the silver chloride. Leads are soldered directly to the

collector grid of nonsilver cathodes or soldered to a piece of copper foil, which has been

stapled to the collector grid.

The anode connection is made by soldering the lead to silver foil, which has been welded

to the anode, or by welding directly to the anode.

Encasement. The battery encasement must effectively rigidize the battery and provide

openings at opposite ends to allow free ingress and egress of electrolyte and corrosion

products.

The periphery of the battery must be sealed in such a manner that the cells contact the

external electrolyte only at the openings provided at the top and bottom of the battery. The

encasement can be accomplished by using premolded pieces, caulking compounds, epoxy

resins, an insulating sheet, or hot-melt resins.

13–16

For single-batteries these precautions are

not necessary.

17.4.2 Leakage Current

All the cells in the immersion- and forced-ﬂow-type batteries operate in a common electro-

lyte. Since the electrolyte is conductive and continuous from cell to cell, conductive paths

exist from each point in a battery to every other point. Current will ﬂow through these

conductive paths to points of different potential. This current is referred to as ‘‘leakage

current’’ and is in addition to the current ﬂowing through the load. Electrodes must be

designed to compensate for these leakage currents.

Leakage currents for a small number of cells can be reduced by increasing the resistance

path from a cell to the common electrolyte or that of the common electrolyte between

adjacent cells. Leakage currents for a large number of cells can be reduced by increasing

the resistance of the common electrolyte external to the individual cells.

By construction the conducting paths from cell to cell are made as long as possible. In

many instances the negative or positive of the battery is connected to an external metal

surface. Leakage currents ﬂow from the battery to this surface. These leakage currents are

controlled by placing a cap containing a slot over the battery openings. If one terminal is

connected to an external conductive surface, the slot in the cap is opened to the electrolyte

17.10 CHAPTER SEVENTEEN

only on that side of the battery. Where neither terminal is connected to an external conductive

surface, either end of the cap may be opened; however only that on one side of the battery

should be opened.

The resistance (ohms) of the slot in the cap may be calculated using the formula

R ⫽ p

where R

⫽ resistance, ⍀

l ⫽ length of slot, cm

⫽ cross-sectional area of slot, cm

p ⫽ resistance of electrolyte for temperature and salinity in which battery is operat-

ing,

⍀ 䡠 cm

For dunk-type batteries the electrolyte continuity from cell to cell is broken when the

electrolyte is absorbed in the separator. The excess is poured off the battery or spun away

from the cells by some external force applied to the battery.

17.4.3 Electrolyte

Seawater-activated batteries are designed to operate in an inﬁnite electrolyte, namely, the

oceans of the world. However, for design, development, and quality control purposes, it is

not practical to use ocean water. Thus it is common practice throughout the industry to use

a simulated ocean water. A commercial product, composed of a blend of all the ingredients

required, simpliﬁes the manufacture of simulated ocean water test solutions.

Dunk-type batteries, activated by pouring the electrolyte into the battery where it is ab-

sorbed by the separator, can utilize water or seawater when the temperature is above freezing.

At lower temperatures special electrolytes can be used. The use of a conducting aqueous

electrolyte will result in faster voltage buildup. However, the introduction of salts in the

electrolyte will increase the rate of self-discharge.

17.5 PERFORMANCE CHARACTERISTICS

17.5.1 General

A summary of the performance characteristics of the major water-activated batteries currently

available is given in Table 17.4.

Voltage versus Current Density. Figures 17.6 and 17.7 are representative voltage versus

current density curves for several water-activated battery systems at 35 and 0

⬚C, respectively,

using a simulated ocean water electrolyte.

Discharge Curves. Discharge curves of the magnesium/ silver chloride, magnesium /

cuprous thiocyanate-sulfur, magnesium/ cuprous iodide-sulfur, and magnesium/ lead chloride

electrochemical systems, discharged continuously through various resistances in simulated

ocean water and high and low temperatures and salinities, are shown in Figs. 17.8 to 17.15.

These data show the advantageous performance of the silver chloride system.

Service Life. The capacities per unit of weight versus the average power output of these

same electrochemical systems, at high and low temperatures and salinities, are shown in Fig.

17.16 and 17.17.

MAGNESIUM WATER-ACTIVATED BATTERIES 17.11

TABLE 17.4 Performance Characteristics of Water-Activated Batteries

Cathode

Silver

chloride

Lead

chloride

Cuprous

iodide

Cuprous

thiocyanate

Cuprous

chloride

Anode Magnesium

Electrolyte Tapwater, seawater, or other conductive aqueous solutions

Open-circuit voltage, V 1.6–17 1.1–1.2 1.5–1.6 1.5–1.6 1.5–1.6

V per cell at 5 mA / cm

2 b

1.42–1.52 0.90–1.06 1.33–1.49 1.24–1.43 1.2–1.4

Activation, s:

⬚C

⬍1 ⬍1 ⬍1 ⬍1

— — — — 1–10

⬚C

45–90 45–90 45–90 45–90

Internal resistance,

⍀

0.1–2 1–4 1–4 1–4 2

Ah/ g cath. theor.

0.187 0.193 0.141 0.220 0.271

Usable capacity, % of

theoretical

60–75 60–75 60–75 60–75 60–75

Wh/kg 100–150 50–80 50–80 50–80 50–80

Wh/ L 180–300 50–120 50–120 50–120 20–200

Operating temperatures,

⬚C

⫺60 to ⫹65

All but cuprous chloride are immersion type. Cuprous chloride is dunk type.

See voltage vs. current density curves.

Battery preconditioned at ⫹55⬚C, then immersed in simulated ocean water of 3.6 wt.%

Electrolyte at room temperature poured into battery and absorbed by separator.

Battery preconditioned at ⫺20⬚C, then immersed in simulated ocean water of 1.5 wt. %.

Depends on battery design.

100% active material.

Following activation at room temperature.

FIGURE 17.6 Representative cell voltages vs. current density at 35⬚C.

17.12 CHAPTER SEVENTEEN

FIGURE 17.7 Representative cell voltages vs. current density at 0⬚C.

FIGURE 17.8 Magnesium / silver chloride seawater-activated cell discharged continuously at 35⬚C

in simulated ocean water, 3.6% salinity.

MAGNESIUM WATER-ACTIVATED BATTERIES 17.13

FIGURE 17.9 Magnesium / silver chloride seawater-activated cell discharged continuously at 0⬚C

in simulated ocean water, 1.5% salinity.

FIGURE 17.10 Magnesium / cuprous thiocyanate seawater-activated cell discharged continuously

at 35⬚C in simulated ocean water, 3.6% salinity.

17.14 CHAPTER SEVENTEEN

FIGURE 17.11 Magnesium / cuprous thiocyanate seawater-activated cell discharged continuously at

0⬚C in simulated ocean water, 1.5% salinity.

FIGURE 17.12 Magnesium / cuprous iodide seawater-activated cell discharged continuously at

35⬚C in simulated ocean water, 3.6% salinity.