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

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METAL / AIR BATTERIES 38.33

FIGURE 38.31 Aluminum dissolved oxygen ﬂat-plate battery (attached

to Woods Hole Oceanographic Institution buoy.) (Courtesy of Alupower,

Inc.)

38.4.2 Aluminum/Air Cells in Alkaline Electrolytes

The concept of operating aluminum/air batteries at high energy and power densities was

described in the early 1970s, but successful commercialization was impeded because of

technological limitations, including the high open-circuit corrosion rate of aluminum alloys

in alkaline electrolyte, the nonavailability of thin, large-dimension air cathodes, and the

difﬁculty of handling and removing the cell reaction products (precipitated aluminum hy-

droxide) to prevent cell clogging.

Signiﬁcant advances have been made in reducing the corrosion of aluminum alloys in

caustic electrolytes.

39,40

An aluminum anode, containing magnesium and tin, has approxi-

mately two orders of magnitude reduction in corrosion current at open circuit and operates

at greater than 98% coulombic efﬁciency over a wide range of current densities. Even at

open circuit, the alloy can remain in a caustic electrolyte with a relatively low rate of self-

discharge. Prior to this development, the amount of hydrogen and heat evolved during open-

circuit stand was usually so high as to require that the electrolyte be drained from the alloy

during this period to prevent it from boiling.

38.34 CHAPTER THIRTY-EIGHT

FIGURE 38.32 Polarization curves of particulate-feed aluminum / air battery. - - -, positive

electrode; ——, negative electrode. (Courtesy of Sorapec.)

FIGURE 38.33 Voltage vs. aluminate concentration. (Courtesy of Eltech Systems.)

Techniques to handle the aluminum so that it can be continuously fed as chips or pellets

into the electrochemical system have also been developed.

One design uses aluminum

particles having diameters of 1 to 5 mm.

The electrode is a pocket whose walls are com-

posed of cadmium-plated expanded steel screen. The electrode is fed by a system which

keeps the cell maintained with an aluminum particulate at an optimum level. Figure 38.32

shows the performance of a cell operating at 50

⬚C using 8N KOH-containing stannate. The

battery, with an electrode area of 360 cm

, was able to deliver a current of 56 A at 1.35 V

for 110 h, with the automatic addition of aluminum every 20 min.

Management of the electrolyte to remove the reaction product from it is required as the

conductivity of the electrolyte decreases with increasing aluminate concentration. As shown

in Fig. 38.33, the voltage decreases if the aluminate is not removed. Several techniques have

been developed to remove the reaction products and are discussed later in this section.

METAL / AIR BATTERIES 38.35

Applications of Alkaline Aluminum/ Air Batteries. The alkaline aluminum/air batteries

being developed cover a wide range of applications from emergency power supplies to ﬁeld-

portable batteries for remote power applications and underwater vehicles. Most of these are

designed as reserve batteries, which are activated before use, or ‘‘mechanically’’ recharged

by replacing the exhausted aluminum anodes.

Reserve Power Units, Standby Battery.

43,44

This is a reserve battery, used with conven-

tional lead-acid batteries, to provide a standby power supply with extended service life. The

aluminum/ air battery is about one-tenth the weight and one-seventh the volume of a lead-

acid battery containing the same energy. The basic elements of the power supply design are

shown in Fig. 38.34. The aluminum/air battery consists of an upper cell stack, a lower

electrolyte reservoir (which is isolated from the stack during periods of nonuse), and auxiliary

systems to pump and cool the electrolyte and circulate air through the battery. The electrolyte

is an 8M solution of potassium hydroxide containing a stannate additive. During operation

the electrolyte becomes increasingly saturated and then supersaturated with potassium alu-

minate. Eventually the conductivity of the electrolyte decreases to the point where the battery

is unable to sustain the load. At that point it has reached the end of its capacity based on

total electrolyte volume (VLD). The electrolyte can be changed at this point, and the dis-

charge continued to the point where the aluminum in the anode is exhausted (ALD). Figure

38.35 shows the performance for a nominal 1200-W battery discharged in the two modes.

Operation in the electrolyte-capacity limited mode will yield a total discharge time of 36 h,

while operation to anode exhaustion, incorporating one electrolyte change, will result in a

total discharge time of 48 h. The overall energy density and speciﬁc energy are greater than

150 Wh /L and 250 Wh /kg.

The control system for this power unit is arranged so that in the event of a power outage,

the lead-acid batteries provide the backup for the ﬁrst 1 to 3 h. As the voltage of the lead-

acid battery begins to fall, the aluminum/air battery is activated by a controller which ini-

tiates the pumping of electrolyte from a reservoir through the aluminum/air cell stack. Once

the aluminum/ air battery reaches full power (about 15 min from activation), it provides full

power to the load and recharges the depleted lead-acid batteries. The electrical characteristics

of the unit are shown in Figure 38.36. The aluminum/air battery has limited restart capability

but can be refurbished by replacing the cell stack and the electrolyte.

Battleﬁeld Power Unit. This power source, called the Special Operations Forces Alu-

minum Air (SOFAL) battery,

was developed as a reserve system to support specialized

military communications equipment in covert ﬁeld operations. The SOFAL weighs approx-

imately 7.3 kg after activation and powers 12 and 24 VDC equipment with pulse currents

up to 10 Amps, continuous drains up to 4 Amps and has a design capacity of 120 Ah. To

minimize weight, this battery is carried to the ﬁeld dry and can be activated with any source

of water.

The SOFAL unit consists of sixteen series-connected cells (Fig. 38.37a) with intercell

connections provided by a printed circuit board. The cell stack, which weighs 3.5 kg dry, is

shown in Fig. 38.37b. Activation of the system is accomplished with 2.5 L of water through

a manifold system to each cell where it dissolves a cast block of stannated potassium hy-

droxide giving a 30% (w/w) KOH solution. After activation, each cell has an open circuit

voltage of 1.7 v or 27.2 v for the cell stack. Electrochemistry of the battery is similar to that

described earlier. Dissolution of the KOH and corrosion of the aluminum provide heat to

operate the system even at low temperature. The unit has been designed to access 1.6 L/

min of air, which provides for low-power operation. If ambient air ﬂow is insufﬁcient, a

small fan, activated by the battery, will provide the required airﬂow and will dissipate excess

heat during high power use. The SOFAL unit provides up to two weeks of service after

activation.

38.36 CHAPTER THIRTY-EIGHT

FIGURE 38.34 Aluminum/ air reserve power unit, standby battery. (Courtesy of Alupower, Inc.)

METAL / AIR BATTERIES 38.37

FIGURE 38.35 Comparison of volume (VLD) vs. anode (ALD) limited discharges

of aluminum / air reserve power unit. (Courtesy of Alupower, Inc.)

FIGURE 38.36 Discharge proﬁle of 6-kW aluminum/ air reserve power unit. (Direct

connection to power system.) (Courtesy of Alupower, Inc.)

38.38 CHAPTER THIRTY-EIGHT

(a)

(b)

(c)

FIGURE 38.37 SOFAL battery. (a) Cell design conﬁguration, (b) Battery block design conﬁg-

uration, (c) Full scale design layout.

METAL / AIR BATTERIES 38.39

The Electronics Module Package (EMP) shown in Fig. 38.37c contains the electronic

components, provides the mounting for an internal rechargeable battery, and houses the fan.

The electronics package contains the power management circuitry to keep the internal sec-

ondary battery fully charged, provides both 24 V and a regulated 12 V output and can be

used to power electronics directly or recharge external batteries. Figure 38.38a shows the

discharge proﬁle of the SOFAL battery on a 2 Amp continuous load on 24 Volt operation.

Figure 38.38b shows the discharge curves for two cells from the SOFAL battery. Cell No.

1 was activated with 8 molar KOH, while cell No. 2 employed KOH pellets in the cell and

was activated with water. Both cells were discharged at 0.5 Amps, and provided 135 Amp-

hours capacity, but cell No. 1 operated at slightly higher voltage, particularly at the end of

discharge.

Following discharge, the cell stack can be replaced to provide a new battleﬁeld power

unit.

(a)

VOLTS

AMPS

HOURS

VOLTS/AMPS

FIGURE 38.38 Sixteen cell SOFAL battery. (a) Constant current 2.0 Ampere discharge.

38.40 CHAPTER THIRTY-EIGHT

(b)

VOLTAGE (VDC)

TIME (Hrs)

CURRENT (A)

FIGURE 38.38 (b) SOFAL test cells on 0.5 Amp resistive discharge (Continued).

Underwater Propulsion. Another area of application for alkaline aluminum batteries is

in self-contained extended-duration power supplies for underwater vehicles such as un-

manned vehicles for submarine and mine surveillance, long-range torpedoes, swimmer de-

livery vehicles, and submarine auxiliary power.

46,47

In these applications, the oxygen can be

carried in pressurized or cryogenic containers, or it can be obtained from the decomposition

of hydrogen peroxide or from oxygen candles. The aluminum/ oxygen system can produce

almost twice as much energy per kilogram of oxygen as a hydrogen/oxygen fuel cell, as

the operating voltage of 1.2 to 1.4 V is almost twice that of the fuel cell. One type of an

aluminum/ oxygen battery for the propulsion of underwater vehicles is shown in Fig. 38.39,

and its characteristics are listed in Table 38.11.

This battery uses a ‘‘self-managing’’ electrolyte system, where the required electrolyte

circulation and precipitation take place within the cell chamber, without the requirement for

external pumps. This has the advantage of allowing the design of battery systems without

electrolyte circulation pumps. There are no shunt currents between cells as each cell is

independent, and there are no electrolyte paths between cells. In addition, the cells can be

conformal with the system which they are designed to power. Figure 38.40 shows the design

of a system using 19-in diameter (48.25-cm) cells. Each cell is about 0.5 in (1.25 cm) thick.

Thermal and concentration gradients and the resulting convection currents within the cell

precipitate the reaction product to the bottom. With this type of system, it is possible to

utilize about 0.8 Ah/cm

of electrolyte. Figure 38.41 gives a discharge curve for a cell which

is exactly one-half the cell shown in Fig. 38.40. The cell was divided down the center to

provide for redundancy. The discharge was carried out at a constant power level of 18 W

and a current density of about 50 mA/ cm

. The ﬁgure shows that the voltage remains

relatively constant, between 1.4 and 1.5 V, for most of the run.

METAL / AIR BATTERIES 38.41

FIGURE 38.39 Aluminum / oxygen power system. (Courtesy of Alupower, Inc.)

TABLE 38.11 Characteristics of Aluminum/ Oxygen Battery

Performance:

Power 2.5 kW

Capacity 100 kWh

Voltage 120 V nominal

Endurance 40 h at full power

Fuel 25-kg aluminum anodes

Oxidant 22 kg oxygen at 4000 lb /in

Buoyancy Neutral, including aluminum

hull section

Time to refuel 3 h

Dimensions:

Mass 360 kg

Battery diameter 470 mm

Hull diameter 533 mm

System length 2235 mm

Performance:

Energy density 265 Wh /L

Speciﬁc energy 265 Wh / kg

38.42 CHAPTER THIRTY-EIGHT

FIGURE 38.40 Self-managing cell system. (Courtesy of Alupower/ Alcan.)

FIGURE 38.41 Discharge curve for self-managing cell. (Courtesy of Alupower/ Alcan.)