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

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23.6 CHAPTER TWENTY-THREE

TABLE 23.4b World Battery Trends-SLI Shipment*

1990 1991

% of total

(1991) 1999

North America 84.9 81.4 35 120

Europe 67.3 69.6 29 79

Asia/ Paciﬁc 53.7 58.8 24 —

Latin American 17.9 18.9 8 —

Africa/ Middle East 10.0

9.6 4 —

Total 233.8 238.3 100 320 est.

*In millions of units.

Source: From Amistadl.

An important aspect of lead is that it is a recoverable resource. It has been estimated that

more than 95% of the batteries sold in the United States are ultimately recycled, and it takes

considerably less energy to recycle lead, a low-melting metal (mp 327.4

⬚C), than to produce

the metals used in other storage battery systems (nickel, iron, zinc, silver, and cadmium) in

battery-grade quality.

23.2 CHEMISTRY

23.2.1 General Characteristics

The lead-acid battery uses lead dioxide as the active material of the positive electrode and

metallic lead, in a high-surface-area porous structure, as the negative active material. The

physical and chemical properties of these materials are listed in Table 23.5.

Typically, a

charged positive electrode contains both

␣

-PbO

(orthorhombic) and

␤

-PbO

(tetragonal).

The equilibrium potential of the

␣

-PbO

is more positive than that of

␤

-PbO

by 0.01 V.

The

␣ form also has a larger, more compact crystal morphology which is less active elec-

trochemically and slightly lower in capacity per unit weight; it does, however, promote longer

cycle life. Neither of the two forms is fully stoichiometric. Their composition can be rep-

resented by PbO

, with x varying between 1.85 and 2.05. The introduction of antimony, even

at low concentrations, in the preparation or cycling of these species leads to a considerable

increase in their performance. The preparation of the active material precursor consists of a

series of mixing and curing operations using leady lead oxide (PbO

⫹ Pb), sulfuric acid,

and water. The ratios of the reactants and curing conditions (temperature, humidity, and

time) affect the development of crystallinity and pore structure. The cured plate consists of

lead sulfate, lead oxide, and some residual lead (

⬍5%). The positive active material, which

is formed electrochemically from the cured plate, is a major factor inﬂuencing the perform-

ance and life of the lead-acid battery. In general the negative, or lead, electrode controls

cold-temperature performance (such as engine starting).

The electrolyte is a sulfuric acid solution, typically about 1.28 speciﬁc gravity or 37%

acid by weight in a fully charged condition.

LEAD-ACID BATTERIES 23.7

TABLE 23.5 Physical and Chemical Properties of Lead and Lead Oxides (PbO

)

Property Lead

␣

-PbO

␤

-PbO

Molecular weight, g/ mol 207.2 239.19 239.19

Composition PbO:

1.94–2.03

PbO:

1.87–2.03

Crystalline form Face-centered

cubic

Rhombic (columbite) Tetragonal (rutile)

Lattice parameters, nm a

⫽ 0.4949 a ⫽ 0.4977

⫽ 0.5948

⫽ 0.5444

⫽ 0.491–0.497

⫽ 0.337–0.340

X-ray density, g /cm

11.34 9.80 ⬃9.80

Practical density at 20

⬚C

(depends on purity), g /cm

11.34 9.1–9.4 9.1–9.4

Heat capacity, cal /deg

mol 6.80 14.87 14.87

Speciﬁc heat, cal/ g 0.0306 0.062 0.062

Electrical resistivity, at 20

⬚C,

␮

⍀/cm

⬃100 ⫻ 10

Electrochemical potential in

4.4M H

at 31.8⬚C, V

0.356

⬃1.709 ⬃1.692

Melting point,

⬚C 327.4

Source: Ref. 5 and others.

As the cell discharges, both electrodes are converted to lead sulfate. The process reverses

on charge:

discharge

⫹

—— —



Pb Pb ⫹ 2eNegative electrode

———



charge

discharge

⫹

⫺

—— —



Pb ⫹ SO PbSO

———



charge

discharge

⫹

—— —



PbO ⫹ 4H ⫹ 2e Pb ⫹ 2H OPositive electrode

———



charge

discharge

⫹

⫺

—— —



Pb ⫹ SO PbSO

———



charge

discharge

—— —



Pb ⫹ PbO ⫹ 2H SO 2PbSO ⫹ 2H OOverall reaction

———

224



charge

As shown, the basic electrode processes in the positive and negative electrodes involve a

dissolution-precipitation mechanism and not a solid-state ion transport or ﬁlm formation

mechanism.

The discharge-charge mechanism, known as double-sulfate reaction, is also

shown graphically in Fig. 23.1.

As the sulfuric acid in the electrolyte is consumed during

discharge, producing water, the electrolyte is an ‘‘active’’ material and in certain battery

designs can be the capacity-limiting material.

As the cell approaches full charge and the majority of the PbSO

has been converted to

Pb or PbO

, the cell voltage on charge becomes greater than the gassing voltage (about 2.39

V per cell) and the overcharge reactions begin, resulting in the production of hydrogen and

oxygen (gassing) and the resultant loss of water.

23.8 CHAPTER TWENTY-THREE

FIGURE 23.1 Discharge and charge reactions of lead-acid cell. (a) Discharge reac-

tions. (b) Charge reactions. (From Mantell.

)

⫹

2H ⫹ 2e → HNegative electrode

⫹

HO⫺ 2e → ⁄

O ⫹ 2HPositive electrode

HO→ H ⫹ ⁄

OOverall reaction

222

In sealed lead-acid cells this reaction is controlled to minimize hydrogen evolution and the

loss of water by recombination of the evolved oxygen with the negative plate (see Sec. 24.2).

The general performance characteristics of the lead-acid cell, during charge and discharge,

are shown in Fig. 23.2. As the cell is discharged, the voltage decreases due to depletion of

material, internal resistance losses, and polarization. If the discharge current is constant, the

voltage under load decreases smoothly to the cutoff voltage and the speciﬁc gravity decreases

in proportion to the Ampere-hours discharged.

An analysis of the behavior of the positive and negative plates can be done by measuring

the voltage between each electrode and a reference electrode, the ‘‘half-cell’’ voltage. Figure

23.3 illustrates this analysis, using a cadmium reference electrode. The industry is, however,

shifting away from cadmium reference electrodes to more stable materials as discussed later

(Sec. 23.2.3).

LEAD-ACID BATTERIES 23.9

FIGURE 23.2 Typical voltage and speciﬁc gravity characteristics of lead-acid cell at constant-

rate discharge and charge.

FIGURE 23.3 Typical charge-discharge curves of

lead-acid cell. (From Mantell.

)

23.10 CHAPTER TWENTY-THREE

Voltage. The nominal voltage of the lead-acid cell is 2 V; the voltage on open circuit is a

direct function of the electrolyte concentration, ranging from 2.125 V for a cell with 1.28

speciﬁc gravity electrolyte to 2.05 V with 1.21 speciﬁc gravity (see Sec. 23.2.2). The end

or cutoff voltage on moderate-rate discharges is 1.75 V per cell, but may range to as low as

1.0 V per cell at extremely high discharge rates at low temperatures.

Speciﬁc Gravity. The selection of speciﬁc gravity used for the electrolyte depends on the

application and service requirements (see Table 23.12). The electrolyte concentration must

be high enough for good ionic conductivity and to fulﬁll electrochemical requirements, but

not so high as to cause separator deterioration or corrosion of other parts of the cell, which

would shorten life and increase self-discharge. The electrolyte concentration is deliberately

reduced in high-temperature climates. During discharge, the speciﬁc gravity decreases in

proportion to the Ampere-hours discharged (Table 23.6). The speciﬁc gravity is thus a means

for checking the state of charge of the battery. On charge, the change in speciﬁc gravity

should similarly be proportional to the Ampere-hour charge accepted by the cell. However,

there is a lag because complete mixing of the electrolyte does not occur until gassing com-

mences near the end of the charge.

TABLE 23.6 Speciﬁc Gravity of Lead-Acid Battery

Electrolytes at Different States of Charge for Various

Designs.*

State of charge

Speciﬁc gravity

ABCD

100% (full charge) 1.330 1.280 1.265 1.225

75% 1.300 1.250 1.225 1.185

50% 1.270 1.220 1.190 1.150

25% 1.240 1.190 1.155 1.115

Discharged 1.210 1.160 1.120 1.0

Assumes ﬂooded cell design.

*Speciﬁc gravity may range from 100 to 150 points between full

charge and discharge depending on cell design: A—electric vehicle

battery; B—traction battery; C—SLI battery; D—stationary battery.

23.2.2 Open-Circuit Voltage Characteristics

The open-circuit voltage for a battery system is a function of temperature and electrolyte

concentration as expressed in the Nernst equation for the lead-acid cell (see also Chap. 2).

␣

HSO

E ⫽ 2.047 ⫹ ln

冉冊

␣

Since the concentration of the electrolyte varies, the relative activities of H

and H

in the Nernst equation change. A graph of the open-circuit voltage versus electrolyte con-

centration at 25

⬚C is given in Fig. 23.4. The plot is fairly linear above 1.10 speciﬁc gravity,

but shows strong deviations at lower concentrations. The open-circuit voltage is also affected

by temperature. The temperature coefﬁcient of the open-circuit voltage of the lead-acid bat-

tery is shown in Fig. 23.5. Where dE/dT is positive, such as above 0.5 Molar H

, the

reversible potential of the system increases with increasing temperature. Below 0.5 M, the

temperature coefﬁcient is negative. Most lead-acid batteries operate above 2 Molar H

(1.120 speciﬁc gravity) and have a thermal coefﬁcient of about ⫹0.2 mV/⬚C.

LEAD-ACID BATTERIES 23.11

FIGURE 23.4 Open-circuit voltage of lead-acid cell as a function of electrolyte speciﬁc

gravity.

FIGURE 23.5 Temperature coefﬁcient of open-circuit voltage

of lead-acid cell as a function of electrolyte speciﬁc gravity.

23.12 CHAPTER TWENTY-THREE

23.2.3 Polarization and Resistive Losses

When a battery is being discharged, the voltage under load is lower than the open-circuit

voltage at the same concentrations of H

and H

O in the electrolyte and Pb or PbO

and

PbSO

in the plates. The thermodynamically stable state for batteries is the discharged state.

Work (charging) must be done to cause the equilibria of the electrochemical reactions to go

toward PbO

in the positive and Pb in the negative. Thus the voltage of the power source

for recharging the lead-acid battery must be higher than the Nernst voltage of the battery on

open circuit.

These deviations from the open-circuit voltage during charge or discharge are due, in

part, to resistive losses in the battery and, in part, to polarization. These losses can be

measured by use of an interrupted discharge, where the IR losses can be estimated by Ohm’s

law (

⌬E/ ⌬I ⫽ R) within a few seconds to a few minutes after the discharge is stopped. The

effect of polarization can take several hours to measure in order for diffusion to allow the

plate interiors to reequilibrate. AC impedance spectroscopy techniques are also of value.

Polarization is more easily measured by use of a reference electrode. The standard reference

of hydrogen on platinum is not practical for most measurements on lead-acid batteries, and

several other sulfate-based reference electrode systems are used. A review of reference elec-

trodes

neglects several very practical sulfate electrodes. Still, a commonly used electrode

for battery maintenance is the cadmium ‘‘stick,’’ but it is not especially stable (

20 mV/

day). Mercury-mercurous sulfate reference electrodes are stable and are available from sev-

eral vendors. A novel Pb/ H

/PbO

reference electrode has been patented.

This electrode

measures the polarization on charge or discharge directly, without need for a correction of

different thermal coefﬁcients of EMF. The change in polarization between the start and the

end of discharge is typically 50 to several hundred mV, and the cell capacity is limited by

the plate group (positive or negative) that has the largest change in polarization during

discharge. When both groups in a cell change about equally, the capacity limitation is more

likely depletion of H

in the electrolyte than depletion of Pb or PbO

in the plates. On

charge, the polarization is a good measure that both positives and negatives have been re-

charged: the plate polarizations change by more than 60 mV between start and end of

recharge. Polarization voltages stabilize at some value when plates are recharged and are

gassing freely.

23.2.4 Self-Discharge

The equilibria of the electrode reactions are normally in the discharge direction since, ther-

modynamically, the discharged state is most stable. The rate of self-discharge [loss of ca-

pacity (charge) when no external load is applied] of the lead-acid cell is fairly rapid, but it

can be reduced signiﬁcantly by incorporating certain design features.

The rate of self-discharge depends on several factors. Lead and lead dioxide are ther-

modynamically unstable in sulfuric acid solutions, and on open circuit, they react with the

electrolyte. Oxygen is evolved at the positive electrode and hydrogen at the negative, at a

rate dependent on temperature and acid concentration (the gassing rate increases with in-

creasing acid concentration) as follows:

PbO ⫹ HSO → PbSO ⫹ HO⫹ ⁄

224 42 2

Pb ⫹ HSO → PbSO ⫹ H

24 4 2

For most positives, the formation of PbSO

by self-discharge is slow, typically much less

than 0.5%/day at 25

⬚C. (Some positives which have been made with nonantimonial grids

can fail by a different mechanism on open circuit, namely, the development of a grid-active

material barrier layer.) The self-discharge of the negative is generally more rapid, especially

if the cell is contaminated with various catalytic metallic ions. For example, antimony lost

LEAD-ACID BATTERIES 23.13

FIGURE 23.6 (a) Capacity retention during stand or storage at 25⬚C. (From Sa-

batino.

)(b) Loss of speciﬁc gravity per day with temperature of a new, fully charged

lead-acid battery with 6% antimonial lead grids. (From Ref. 10.)

from the positive grids by corrosion can diffuse to the negative, where it is deposited, re-

sulting in a ‘‘local action’’ discharge cell which converts some lead active material to PbSO

New batteries with lead-antimony grids lose about 1% of charge per day at 25

⬚C, but the

charge loss increases by a factor of 2 to 5 as the battery ages. Batteries with nonantimonial

lead grids lose less than 0.5% of charge per day regardless of age. This is illustrated in Fig.

23.6a.

Maintenance-free and charge-retention-type batteries, where the self-discharge rate

must be minimized, use low-antimony or antimony-free alloy (such as calcium-lead) grids.

However, because of other beneﬁcial effects of antimony, its complete elimination may not

be desirable, and low-antimony–lead alloys are a useful compromise.

Self-discharge is temperature-dependent, as shown in Fig. 23.6b.

The graph shows the

fall in speciﬁc gravity per day of a new fully charged battery with 6% antimonial lead grids.

Self-discharge can thus be minimized by storing batteries at temperatures between 5 and

⬚C.

23.14 CHAPTER TWENTY-THREE

TABLE 23.7 Properties of Sulfuric Acid Solutions*

Speciﬁc

gravity

⬚C

25⬚C

Temperature

coeff.

␣

Wt., % Vol., % Mol /L

Freezing

point,

⬚C

Elecro-

chemical

equivalent

(per liter of

acid), Ah

1.00 1.000 — 0 0 0 0 0

1.05 1.049 33 7.3 4.2 0.82

⫺3.3 22

1.10 1.097 48 14.3 8.5 1.65

⫺7.7 44

1.15 1.146 60 20.9 13.0 2.51

⫺15 67

1.20 1.196 68 27.2 17.7 3.39

⫺27 90

1.25 1.245 72 33.2 22.6 4.31

⫺52 115

1.30 1.295 75 39.1 27.6 5.26

⫺70 141

1.35 1.345 77 44.7 32.8 6.23

⫺49 167

1.40 1.395 79 50.0 38.0 7.21

⫺36

1.45 1.445 82 55.0 43.3 8.2

⫺29

1.50 1.495 85 59.7 48.7 9.2

⫺29

*To calculate the speciﬁc gravity for any temperature, ⬚C, SG (t) ⫽ SG (15⬚C) ⫹

␣

⫻ 10

⫺

(15 ⫺ t).

FIGURE 23.7a Freezing points of sulfuric acid solutions at various speciﬁc gravities.

The inﬂection points result from the different water to SO

hydration ratios.

23.2.5 Characteristics and Properties of Sulfuric Acid

The major characteristics and properties of the sulfuric acid electrolyte, as they apply to the

operation of the lead-acid battery, are listed in Table 23.7. The freezing points of sulfuric

acid solutions at various concentrations are also plotted in Fig. 23.7a. The freezing point of

aqueous sulfuric acid solutions varies signiﬁcantly with concentration. Batteries must there-

fore be designed so that the electrolyte concentration is above the value at which the elec-

trolyte would freeze when exposed to the anticipated cold. Alternatively, the battery can be

insulated or heated so that it remains above the electrolyte freezing temperature.

Figure 23.7b shows the speciﬁc resistivity of sulfuric acid solutions at various speciﬁc

gravities as a function of temperature from

⫺40 to 40⬚C.

LEAD-ACID BATTERIES 23.15

FIGURE 23.7b Speciﬁc resistivity of sulfuric acid solutions at various speciﬁc

gravities and temperatures.